Update id_mode_to_cifs_acl to take a kuid_t and a kgid_t.

Replace NO_CHANGE_32 with INVALID_UID and INVALID_GID, and tests for
NO_CHANGE_32 with uid_valid and gid_valid.

Carefully unpack the value returned from request_key.  memcpy the
value into the expected type.  The convert the uid/gid into a
kuid/kgid.  And then only if the result is a valid kuid or kgid update
fuid/fgid.

Cc: Steve French <smfrench@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>