sfc: replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

`desc` is expected to be NUL-terminated as evident by the manual
NUL-byte assignment. Moreover, NUL-padding does not seem to be
necessary.

The only caller of efx_mcdi_nvram_metadata() is
efx_devlink_info_nvram_partition() which provides a NULL for `desc`:
|       rc = efx_mcdi_nvram_metadata(efx, partition_type, NULL, version, NULL, 0);

Due to this, I am not sure this code is even reached but we should still
favor something other than strncpy.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP/linux#90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231012-strncpy-drivers-net-ethernet-sfc-mcdi-c-v1-1-478c8de1039d@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: JustinStitt

drivers/net/ethernet/sfc/mcdi.c

@@ -2205,10 +2205,9 @@ int efx_mcdi_nvram_metadata(struct efx_nic *efx, unsigned int type,
 				goto out_free;
 			}
 
-			strncpy(desc,
+			strscpy(desc,
 				MCDI_PTR(outbuf, NVRAM_METADATA_OUT_DESCRIPTION),
 				MC_CMD_NVRAM_METADATA_OUT_DESCRIPTION_NUM(outlen));
-			desc[MC_CMD_NVRAM_METADATA_OUT_DESCRIPTION_NUM(outlen)] = '\0';
 		} else {
 			desc[0] = '\0';
 		}