btrfs: cleanup btrfs_setxattr_trans and drop transaction parameter

Previous patch made sure that btrfs_setxattr_trans() is called only when
transaction NULL.  Clean up btrfs_setxattr_trans() and drop the
parameter.

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