fix breakage caused by d_find_alias() semantics change

"VFS: don't keep disconnected dentries on d_anon" had a non-trivial
side-effect - d_unhashed() now returns true for those dentries,
making d_find_alias() skip them altogether.  For most of its callers
that's fine - we really want a connected alias there.  However,
there is a codepath where we relied upon picking such aliases
if nothing else could be found - selinux delayed initialization
of contexts for inodes on already mounted filesystems used to
rely upon that.

Cc: stable@kernel.org # f1ee6162 "VFS: don't keep disconnected dentries on d_anon"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>