Be more strict when selecting between snapshot/regular mount.

Reclaimed vnode type is VBAD, so succesful comparision like
devvp->v_type != VREG does not imply that the devvp references
snapshot, it might be due to a reclaimed vnode. Explicitely check the
vnode type.

In the the most important case of ffs_blkfree(), the devfs vnode is
locked and its type is stable. In other cases, if the vnode is
reclaimed right after the check, hopefully the buffer methods return
right error codes.

Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks