In other words, deny multiple read-only mounts of the same device.
Shared read-only mounts should theoretically be possible, but,
unfortunately, can not be implemented correctly using current
buffer cache code/interface and results in an eventual system crash.
Also, using nullfs seems to be a more efficient way to achieve the same
goal.
This gets us back to where we were before GEOM and where other BSDs are.
Submitted by: pjd (idea for checking for shared mounting)
Discussed with: phk, pjd
Silence from: fs@, geom@
MFC after: 2 weeks
8f128ff559