btrfs: fix function name in comment for btrfs_remove_ordered_extent()

Due to a refactoring introduced by commit 53d9981c

 ("btrfs: split
btrfs_alloc_ordered_extent to allocation and insertion helpers"), the
function btrfs_alloc_ordered_extent() was renamed to
alloc_ordered_extent(), so the comment at btrfs_remove_ordered_extent()
is no longer very accurate. Update the comment to refer to the new
name "alloc_ordered_extent()".

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>