btrfs: defrag: don't defrag extents which are already at max capacity

[BUG]
For compressed extents, defrag ioctl will always try to defrag any
compressed extents, wasting not only IO but also CPU time to
compress/decompress:

   mkfs.btrfs -f $DEV
   mount -o compress $DEV $MNT
   xfs_io -f -c "pwrite -S 0xab 0 128K" $MNT/foobar
   sync
   xfs_io -f -c "pwrite -S 0xcd 128K 128K" $MNT/foobar
   sync
   echo "=== before ==="
   xfs_io -c "fiemap -v" $MNT/foobar
   btrfs filesystem defrag $MNT/foobar
   sync
   echo "=== after ==="
   xfs_io -c "fiemap -v" $MNT/foobar

Then it shows the 2 128K extents just get COW for no extra benefit, with
extra IO/CPU spent:

    === before ===
    /mnt/btrfs/file1:
     EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
       0: [0..255]:        26624..26879       256   0x8
       1: [256..511]:      26632..26887       256   0x9
    === after ===
    /mnt/btrfs/file1:
     EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
       0: [0..255]:        26640..26895       256   0x8
       1: [256..511]:      26648..26903       256   0x9

This affects not only v5.16 (after the defrag rework), but also v5.15
(before the defrag rework).

[CAUSE]
From the very beginning, btrfs defrag never checks if one extent is
already at its max capacity (128K for compressed extents, 128M
otherwise).

And the default extent size threshold is 256K, which is already beyond
the compressed extent max size.

This means, by default btrfs defrag ioctl will mark all compressed
extent which is not adjacent to a hole/preallocated range for defrag.

[FIX]
Introduce a helper to grab the maximum extent size, and then in
defrag_collect_targets() and defrag_check_next_extent(), reject extents
which are already at their max capacity.

Reported-by: Filipe Manana <fdmanana@suse.com>
CC: stable@vger.kernel.org # 5.16
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>