glibc: Suppress GCC -Os warning on user2netname for sunrpc

glibc: Suppress GCC -Os warning on user2netname for sunrpc

When building with GCC -Os, a warning is triggered indicating that sprintf might overflow.

Error:
  netname.c: In function 'user2netname':
  netname.c:51:28: error: '%s' directive writing up to 255 bytes into a
  region of size between 239 and 249 [-Werror=format-overflow=]
     51 |   sprintf (netname, "%s.%d@%s", OPSYS, uid, dfltdom);
        |                            ^~               ~~~~~~~
  netname.c:51:3: note: 'sprintf' output between 8 and 273 bytes into a
  destination of size 256
     51 |   sprintf (netname, "%s.%d@%s", OPSYS, uid, dfltdom);
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

However the code does test prior the sprintf call that dfltdom plus
the required extra space for OPSYS, uid, and extra character will not
overflow and return 0 instead.

Upstream-patch: https://github.com/bminor/glibc/commit/6128e82ebe973163d2dd614d31753c88c0c4d645

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Nikhil R <nikhilr5@kpit.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>