btrfs: fix failure of tests that use defrag on btrfs-progs v6.0+

btrfs: fix failure of tests that use defrag on btrfs-progs v6.0+

Starting with btrfs-progs v6.0, the defrag command now prints to stdout
the full path of the files it processes. This makes test cases btrfs/021
and btrfs/256 fail because they don't expect any output from the defrag
command.

The change happened with the following commit in btrfs-progs:

  dd724f21803d ("btrfs-progs: add logic to handle LOG_DEFAULT messages")

So update the tests to ignore the stdout of the defrag command.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>