cifs: use helpers when parsing uid/gid mount options and validate them

Use the nice helpers to initialize and the uid/gid/cred_uid when passed as mount arguments.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Acked-by: Pavel Shilovsky <pshilovsky@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>