ksmbd: fix -Wstringop-truncation warnings

Kernel test bot reports the following warnings:

   In function 'ndr_write_string',
       inlined from 'ndr_encode_dos_attr' at fs/ksmbd/ndr.c:136:3:
>> fs/ksmbd/ndr.c:70:2: warning: 'strncpy' destination unchanged after
copying no bytes [-Wstringop-truncation]
      70 |  strncpy(PAYLOAD_HEAD(n), value, sz);
         |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   In function 'ndr_write_string',
       inlined from 'ndr_encode_dos_attr' at fs/ksmbd/ndr.c:134:3:
>> fs/ksmbd/ndr.c:70:2: warning: 'strncpy' output truncated before
terminating nul copying as many bytes from a string as its length
[-Wstringop-truncation]
      70 |  strncpy(PAYLOAD_HEAD(n), value, sz);
         |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   fs/ksmbd/ndr.c: In function 'ndr_encode_dos_attr':
   fs/ksmbd/ndr.c:134:3: note: length computed here
     134 |   ndr_write_string(n, hex_attr, strlen(hex_attr));
         |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>