fix typo in assignment of fs default overflow gid

fix typo in assignment of fs default overflow gid

The patch remains without practical effect since both macros carry
identical values.  Still, it might become a problem in the future if
(for whatever reason) the default overflow uid and gid differ.  The
DEFAULT_FS_OVERFLOWGID macro was previously unused.

Signed-off-by: Wolffhardt Schwabe <wolffhardt.schwabe@fau.de>
Signed-off-by: Anatoliy Cherepantsev <anatoliy.cherepantsev@fau.de>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>