g_vfs_open: allow only one mount per device vnode

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

0 files changed, 0 insertions, 0 deletions