vfs_mount_destroy waiting for this ref to hit 0. We don't print an
error if we are rebooting as the root mount always retains some refernces
by init proc.
- Acquire a mnt ref for every vnode allocated to a mount point. Drop this
ref only once vdestroy() has been called and the mount has been freed.
- No longer NULL the v_mount pointer in delmntque() so that we may release
the ref after vgone() has been called. This allows us to guarantee
that the mount point structure will be valid until the last vnode has
lost its last ref.
- Fix a few places that rely on checking v_mount to detect recycling.
Sponsored by: Isilon Systems, Inc.
MFC After: 1 week
04f6d3effa