btrfs/205: Ignore output of chattr command

btrfs/205: Ignore output of chattr command

With newer kernels, containing upstream commit f37c563bab42 ("btrfs:
add missing check for nocow and compression inode flags") chattr would
produce an error :

  /usr/bin/chattr: Invalid argument while setting flags on /media/scratch/foo1

That's due to the aforementioned commit making the checks stricter. This
is not critical for the purpose of the test so it can be safely
ignored.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>