mount: fix list logic in update_mtab

mount: fix list logic in update_mtab

If mtab does not contain the new entry, then only mc0->prev is updated
to point to absent, but not the old mc0->prev's nxt pointer.  Because
we then use the nxt pointers to write the new mtab, absent is not
added to the new mtab.

(Note: fortunately, the mount doesn't use the update_mtab() for new
mounts, but for remount, move and umount only -- kzak)

If mtab is empty, absent->prev should be set to mc0, and not
mc0->prev, as it will be NULL.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Martin Schlemmer <azarah@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>