ibmvnic: Use strscpy() instead of strncpy()

ibmvnic: Use strscpy() instead of strncpy()

Since these strings are expected to be NUL-terminated and the buffers
are exactly sized (in vnic_client_data_len()) with no padding, strncpy()
can be safely replaced with strscpy() here, as strncpy() on
NUL-terminated string is considered deprecated[1]. This has the
side-effect of silencing a -Warray-bounds warning due to the compiler
being confused about the vlcd incrementing:

In file included from ./include/linux/string.h:253,
                 from ./include/linux/bitmap.h:10,
                 from ./include/linux/cpumask.h:12,
                 from ./include/linux/mm_types_task.h:14,
                 from ./include/linux/mm_types.h:5,
                 from ./include/linux/buildid.h:5,
                 from ./include/linux/module.h:14,
                 from drivers/net/ethernet/ibm/ibmvnic.c:35:
In function '__fortify_strncpy',
    inlined from 'vnic_add_client_data' at drivers/net/ethernet/ibm/ibmvnic.c:3919:2:
./include/linux/fortify-string.h:39:30: warning: '__builtin_strncpy' offset 12 from the object at 'v
lcd' is out of the bounds of referenced subobject 'name' with type 'char[]' at offset 12 [-Warray-bo
unds]
   39 | #define __underlying_strncpy __builtin_strncpy
      |                              ^
./include/linux/fortify-string.h:51:9: note: in expansion of macro '__underlying_strncpy'
   51 |  return __underlying_strncpy(p, q, size);
      |         ^~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/ibm/ibmvnic.c: In function 'vnic_add_client_data':
drivers/net/ethernet/ibm/ibmvnic.c:3883:7: note: subobject 'name' declared here
 3883 |  char name[];
      |       ^~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings

Cc: Dany Madden <drt@linux.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Cc: Thomas Falcon <tlfalcon@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>