hamradio: replace deprecated strncpy with strscpy_pad

hamradio: replace deprecated strncpy with strscpy_pad

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 both hi.data.modename and hi.data.drivername to be
NUL-terminated based on its usage with sprintf:
|       sprintf(hi.data.modename, "%sclk,%smodem,fclk=%d,bps=%d%s",
|               bc->cfg.intclk ? "int" : "ext",
|               bc->cfg.extmodem ? "ext" : "int", bc->cfg.fclk, bc->cfg.bps,
|               bc->cfg.loopback ? ",loopback" : "");

Note that this data is copied out to userspace with:
|       if (copy_to_user(data, &hi, sizeof(hi)))
... however, the data was also copied FROM the user here:
|       if (copy_from_user(&hi, data, sizeof(hi)))

Considering the above, a suitable replacement is strscpy_pad() as it
guarantees NUL-termination on the destination buffer while also
NUL-padding (which is good+wanted behavior when copying data to
userspace).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231016-strncpy-drivers-net-hamradio-baycom_epp-c-v2-1-39f72a72de30@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>