vfs: make vfscount refcounting handle possible kld unload

Make the refcounting occur outside any code that might deref a pointer
to struct vfsconf.

Increment vfc_refcount in vfs_byname(), under vfsconf_lock.
Unref either on early mount failure, before struct mount is
instantiated, or in vfs_mount_destroy().

Reported and reviewed by: kevans
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week