userfaultfd: use secure anon inodes for userfaultfd

This change gives userfaultfd file descriptors a real security
context, allowing policy to act on them.

Signed-off-by: Daniel Colascione <dancol@google.com>
[LG: Remove owner inode from userfaultfd_ctx]
[LG: Use anon_inode_getfd_secure() in userfaultfd syscall]
[LG: Use inode of file in userfaultfd_read() in resolve_userfault_fork()]
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>