btrfs: refactor btrfs_set_prop and add btrfs_set_prop_trans

btrfs_set_prop() takes transaction pointer as the first argument,
however in ioctl.c for the purpose of setting the compression property,
we call btrfs_set_prop() with NULL transaction pointer. Down in
the call chain  btrfs_setxattr() starts transaction to update the
attribute and also to update the inode.

So for clarity, create btrfs_set_prop_trans() with no transaction
pointer as argument, in preparation to start transaction here instead of
doing it down the call chain at btrfs_setxattr().

Also now the btrfs_set_prop() is a static function.

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