Allow the MNT_FORCE flag to be passed through to an initial mount.

When doing an initial mount(8) with its -f (force) flag, the MNT_FORCE
flag is not passed through to the underlying filesystem mount routine.
MNT_FORCE is only passed through on later updates to an existing
mount. With this commit the MNT_FORCE flag is now passed through on the
initial mount.

Sanity check: kib
Sponsored by: Netflix