fs-util: add new unlinkat_deallocate() helper

This new helper not only removes a file from a directory but also
ensures its space on disk is deallocated, by either punching a hole over
the full file or truncating the file afterwards if the file's link
counter is 0. This is useful in "vacuuming" algorithms to ensure that
client's can't keep the disk space the vacuuming is supposed to recover
pinned simply by keeping an fd open to it.