IB/qib: Replace deprecated strncpy

IB/qib: Replace deprecated strncpy

`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 know `txselect_list` is expected to be NUL-terminated based on its
use in `param_get_string()`:
| int param_get_string(char *buffer, const struct kernel_param *kp)
| {
| 	const struct kparam_string *kps = kp->str;
| 	return scnprintf(buffer, PAGE_SIZE, "%s\n", kps->string);
| }

Note that `txselect_list` is assigned to `kp_txselect`'s string field:
| static struct kparam_string kp_txselect = {
| 	.string = txselect_list,
| 	.maxlen = MAX_ATTEN_LEN
| };

Wherein it is then assigned the set and get methods:
| module_param_call(txselect, setup_txselect, param_get_string,
| 		  &kp_txselect, S_IWUSR | S_IRUGO);

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
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230921-strncpy-drivers-infiniband-hw-qib-qib_iba7322-c-v1-1-373727763f5b@google.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>