[Feature] - Reduce syscall overhead of serving files

[lunixbochs]

Recorded with perf trace -p $(pgrep versitygw) and then fetching one file with GetObject

// AclParser middleware
( 0.025 ms): newfstatat(dfd: CWD, filename: 0x32c2a0, statbuf: 0xc0001c66b8)
( 0.031 ms): getxattr(pathname: 0x32c2b8, name: 0x32c2c0, value: 0xc000596000, size: 1024)

// _, err := os.Stat(bucket) in Posix.GetBucketAcl
( 0.009 ms): newfstatat(dfd: CWD, filename: 0x32c2e0, statbuf: 0xc0001c6858)

// fi, err := os.Stat(objPath) in Posix.GetObject
( 0.020 ms): newfstatat(dfd: CWD, filename: 0x1cade0, statbuf: 0xc0001c6928)

// Posix.loadUserMetaData, called from Posix.GetObject
( 0.016 ms): listxattr(pathname: 0x1cae40, list: 0x2036c40)

// b, _ := p.meta.RetrieveAttribute(nil, bucket, object, contentTypeHdr) in Posix.GetObject
( 0.017 ms): getxattr(pathname: 0x1caea0, name: 0x32c300, value: 0xc000596400, size: 1024)

// b, _ = p.meta.RetrieveAttribute(nil, bucket, object, contentEncHdr) in Posix.GetObject
( 0.016 ms): getxattr(pathname: 0x1caf00, name: 0x5400c0, value: 0xc000596800, size: 1024)

// os.Open(objPath) in Posix.GetObject
( 0.017 ms): openat(dfd: CWD, filename: 0x1caf30, flags: RDONLY|CLOEXEC)

4 of the 8 syscalls can be removed here with no downsides:

• os.Stat(bucket) in GetBucketAcl can be moved after p.meta.RetrieveAttribute on error, though RetrieveAttribute may itself even return a fs.ErrNotExist already.
• os.Stat(bucket) in GetObject will currently never be reached for a nonexistent bucket, because the middleware is already raising this error, so this syscall is maybe just redundant. Even if you want defensive coding, we can still avoid the syscall here by only statting the bucket if the file itself is not found, to possibly upgrade a "file not found" error to a "bucket not found" error.
• both p.meta.RetrieveAttribute calls above seem redundant, I think the metadata should already be retrieved in userMetaData from the Posix.loadUserMetaData method? so we can grab it from there iff it exists instead of making extra syscalls when it doesn't exist

Impact:

- ( 0.025 ms): newfstatat(dfd: CWD, filename: 0x32c2a0, statbuf: 0xc0001c66b8)
  ( 0.031 ms): getxattr(pathname: 0x32c2b8, name: 0x32c2c0, value: 0xc000596000, size: 1024)
- ( 0.009 ms): newfstatat(dfd: CWD, filename: 0x32c2e0, statbuf: 0xc0001c6858)
  ( 0.020 ms): newfstatat(dfd: CWD, filename: 0x1cade0, statbuf: 0xc0001c6928)
  ( 0.016 ms): listxattr(pathname: 0x1cae40, list: 0x2036c40)
- ( 0.017 ms): getxattr(pathname: 0x1caea0, name: 0x32c300, value: 0xc000596400, size: 1024)
- ( 0.016 ms): getxattr(pathname: 0x1caf00, name: 0x5400c0, value: 0xc000596800, size: 1024)
  ( 0.017 ms): openat(dfd: CWD, filename: 0x1caf30, flags: RDONLY|CLOEXEC)

This is a vaguely 44% ~(151us -> 84us) syscall overhead reduction per GetObject. Could be worth shaving that further to ~53us (65% reduction) by caching the bucket ACL getxattr with like a 1s ttl, which will amortize it out completely under heavy load.

[lunixbochs]

In trying to reduce this I noticed os.Stat(bucket) is actually responsible for a lot of code across the posix endpoints - is there some way you'd feel comfortable removing almost all of the redundant calls to os.Stat(bucket) for bucket exists and finding another way to make sure it's hit in GetBucketAcl or via the middleware at an appropriate time for each backend? Almost 6% of the code in posix.go is redundant os.Stat(bucket) calls (because it was already called in the acl middleware)