Optim stat()

[kcgthb]

With fd.

[thiell]

c7c4dbd makes use of the fd provided by FUSE in getattr() if the file is open, so in this case, at each getattr we avoid:

• a lookup which takes a lock on migratefs' inode cache and does a check for the file in all layers
• a fstatat using the full path (we still need to do a fstat() by fd only)

Reading a file should now be lockless, so I expect this change to provide a performance improvement on read in multi-threaded mode, especially under mixed workload. This is worth a 0.3.2.