mount.cifs: fix test for strtoul failure in mount.cifs

mount.cifs: fix test for strtoul failure in mount.cifs

It currently test to see if errno == -EINVAL and whether the endptr
is '\0'. That's not correct however. What we really want it to do is
check to see if any error occurred by setting errno to 0 before the
conversion. If one did, then try to treat the value as a name.

Also fix a bogus compiler warning about cruid being uninitialized.

Reported-by: Jian Li <jiali@redhat.com>
Signed-off-by: Jeff Layton <jlayton@samba.org>