Fix "mount -uw /" when the filesystem type doesn't match.

This basically makes "mount -uw /" work when the filesystem
mounted on / is NFS, but the one configured in fstab(5) is UFS,
which can happen when you forget to modify fstab.

Note that the whole special case ("else if (argv[0][0] == '/'")
is probably not needed anyway. I'll take a look at removing it
altogether; for now this is a minimally intrusive fix.

MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11323