dmuserd: Simple dm-user daemon

This provides a block device via dm-user, with all accesses backed by
in-memory storage.  It's essentially the same as what I have in
selftests, with the kselftests stuff removed so it'll build in Android.

Test: mkfs.f2fs, dd, fsck.f2fs
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: I68515d6e9001c2f6d199d394e67ebe528b382406