SUNRPC: use cmpxchg64() in gss_seq_send64_fetch_and_inc()

The newly introduced gss_seq_send64_fetch_and_inc() fails to build on
32-bit architectures:

net/sunrpc/auth_gss/gss_krb5_seal.c:144:14: note: in expansion of macro 'cmpxchg'
   seq_send = cmpxchg(&ctx->seq_send64, old, old + 1);
              ^~~~~~~
arch/x86/include/asm/cmpxchg.h:128:3: error: call to '__cmpxchg_wrong_size' declared with attribute error: Bad argument size for cmpxchg
   __cmpxchg_wrong_size();     \

As the message tells us, cmpxchg() cannot be used on 64-bit arguments,
that's what cmpxchg64() does.

Fixes: 571ed1fd ("SUNRPC: Replace krb5_seq_lock with a lockless scheme")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>