9p: Modify struct 9p_fid to use a kuid_t not a uid_t

Change struct 9p_fid and it's associated functions to
use kuid_t's instead of uid_t.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>