fs/9p: fixes regressions in 6.9

This series contains a reversion of one of the original 6.9
patches which seems to have been the cause of most of the
instability.  It also incorporates several fixes to legacy
support and cache fixes.

There are few additional changes to improve stability,
but I want another week of testing before sending them
upstream.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
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