cifs: Use BUILD_BUG_ON to validate uids and gids are the same size

The assumption that sizeof(uid_t) is the same as sizeof(gid_t) is
completely reasonable but since we can verify the condition at
compile time.

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