UFS: stop reusing the vnode for reallocated inode.

In ffs_valloc(), force reclaim existing vnode on inode reuse, instead
of trying to re-initialize the same vnode for new purposes. This is
done in preparation of changes to the vp->v_object lifecycle handling.

A new FFSV_REPLACE flag to ffs_vgetf() directs the function to
vgone(9) the vnode if found in vfs hash, instead of returning it.

Reviewed by: markj, mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D21412