btrfs: simplify the send_in_progress check in btrfs_delete_subvolume

Only when send_in_progress, we have to do something different such as
btrfs_warn() and return -EPERM. Therefore, we could check
send_in_progress first and process error handling, after the
root_item_lock has been got.

Just for better readability. No functional change.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>