fs/9p: remove writeback fid and fix per-file modes

This patch removes the creating of an additional writeback_fid
for opened files.  The patch addresses problems when files
were opened write-only or getattr on files with dirty caches.

This patch also incorporates information about cache behavior
in the fid for every file.  This allows us to reflect cache
behavior from mount flags, open mode, and information from
the server to inform readahead and writeback behavior.

This includes adding support for a 9p semantic that qid.version==0
is used to mark a file as non-cachable which is important for
synthetic files.  This may have a side-effect of not supporting
caching on certain legacy file servers that do not properly set
qid.version.  There is also now a mount flag which can disable
the qid.version behavior.

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