fs/9p: fix type mismatch in file cache mode helper

There were two flags (s_flags and s_cache) which had incorrect signed
type in the parameters of the file cache mode helper function.

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