Fix function is_btrfs to cover <path|device> correctly.

[develroo]

Currently, in the functions is_btrfs, stat -f will incorrectly report device paths because /dev/ is a virtual filesystem and classified as tmpfs.

Eg.

stat -f /dev/sdb 
  File: "/dev/sdb"
    ID: eb91af7d7bda02dd Namelen: 255     Type: tmpfs
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 2012292    Free: 2012292    Available: 2012292
Inodes: Total: 2012292    Free: 2011839

This is correct behaviour for stat as according to the coreutils info docs:

"stat does not search for specified device nodes in the file system list, instead operating on them directly"

As the BTRFS documentation specifies <path|device> in all the commands, this is obviously undesirable behaviour for the is_btrfs function.

This patch uses df to work around the virtual nature of devices in Linux, while still retaining compatibility with paths.

Hope this helps.

[TheGreatMcPain]

I tried this patch on my system which uses RAID5.

Unfortunately this only works for the first device in the array, since df only sees the first disk.

[TheGreatMcPain]

I think we can change is_btrfs() to this.

is_btrfs() {
    btrfs device stats $1 &>/dev/null
}