vfs: add helpers to check r/o bind mounts

Patches 1-3 are cleanups/bugfixes preceding the main series.  The
description is in 4/13.  This is against latest git.

<rant>

R/O bind mounts patches have been reviewed numerous times.  Still a
relatively large number of places remained where checking for R/O
mounts was missed.

Then I did this series, which basically guarantees, that that cannot
happen.  Al rejected it, and rather fixed some of the remaining
places.  He still missed several, which sort of proves my point.

I think it's totally pointless to continue trying to fix the symptoms
instead of getting at the root of the problem.

I know that VFS interfaces are a sensitive question, but it would be
nice it we could have some sanity back in this discussion.

</rant>

Thanks,
Miklos

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