Before mounting a new mfs on /tmp, we check if there already is one.
However, the dupe check only takes /dev/md[0-9]* into account, while the
default mfs type these days is tmpfs. Rewrite it to look for tmpfs as
well.

Note that the dupe check is redundant in the tmpmfs=auto case, but we
leave moving it for later.

PR: 182035
MFC after: 1 week