fs/9p: mitigate inode collisions

Detect and mitigate inode collsions that now occur since we
fixed 9p generating duplicate inode structures.  Underlying
cause of these appears to be a race condition between reuse
of inode numbers in underlying file system and cleanup of
inode numbers in the client.  Enabling caching
makes this much more likely to happen as it increases cleanup
latency due to writebacks.

Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>