s390/qeth: replace deprecated strncpy with strscpy

s390/qeth: 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.

We expect new_entry->dbf_name to be NUL-terminated based on its use with
strcmp():
|       if (strcmp(entry->dbf_name, name) == 0) {

Moreover, NUL-padding is not required as new_entry is kzalloc'd just
before this assignment:
|       new_entry = kzalloc(sizeof(struct qeth_dbf_entry), GFP_KERNEL);

... rendering any future NUL-byte assignments (like the ones strncpy()
does) redundant.

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
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Thorsten Winkler <twinkler@linux.ibm.com>
Tested-by: Thorsten Winkler <twinkler@linux.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231023-strncpy-drivers-s390-net-qeth_core_main-c-v1-1-e7ce65454446@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>