fs/9p: fix typo in comparison logic for cache mode

There appears to be a typo in the comparison statement for the logic
which sets a file's cache mode based on mount flags.

Cc: stable@vger.kernel.org
Fixes: 1543b4c5 ("fs/9p: remove writeback fid and fix per-file modes")
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>