NFSD: Fix strncpy() fortify warning

In function ‘strncpy’,
    inlined from ‘nfsd4_ssc_setup_dul’ at /home/cel/src/linux/manet/fs/nfsd/nfs4proc.c:1392:3,
    inlined from ‘nfsd4_interssc_connect’ at /home/cel/src/linux/manet/fs/nfsd/nfs4proc.c:1489:11:
/home/cel/src/linux/manet/include/linux/fortify-string.h:52:33: warning: ‘__builtin_strncpy’ specified bound 63 equals destination size [-Wstringop-truncation]
   52 | #define __underlying_strncpy    __builtin_strncpy
      |                                 ^
/home/cel/src/linux/manet/include/linux/fortify-string.h:89:16: note: in expansion of macro ‘__underlying_strncpy’
   89 |         return __underlying_strncpy(p, q, size);
      |                ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>