btrfs: remove list_empty() check at warn_about_uncommitted_trans()

btrfs: remove list_empty() check at warn_about_uncommitted_trans()

At warn_about_uncommitted_trans(), there's no need to check if the list
is empty and return, because list_for_each_entry_safe() is safe to call
for an empty list, it simply does nothing. So remove the check.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>