btrfs: open code check_barrier_error()

check_barrier_error() is almost a single line function, and just calls
btrfs_check_rw_degradable(). Instead, open code it.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>