Handle ro remount of tmpfs rw mount.

[kib]

Summary

In particular:

• suspend the mount around vflush() to avoid new writes come after the vnode is processed
• flush pending metadata updates (mostly node times)
• remap all rw mappings of files from the mount into ro (this is not clear to me how to handle best, might be revoke the mapping at all)
• disable metadata updates after the mp is ro

[kib]

At the moment this is FYI, the patch is the work in progress. Most important, it handles things I know are broken already, but complete audit of tmpfs is needed to identify the rest of it.

[sobomax]

Looks good to my VFS-naive eye. Thanks!

[kib]

Update diff to the fixed committable version.