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 [1]
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://lore.kernel.org/r/2023[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/hamradio/baycom_epp.c

···
 		return 0;
 
 	case HDLCDRVCTL_DRIVERNAME:
-		strncpy(hi.data.drivername, "baycom_epp", sizeof(hi.data.drivername));
+		strscpy_pad(hi.data.drivername, "baycom_epp", sizeof(hi.data.drivername));
 		break;
 		
 	case HDLCDRVCTL_GETMODE:
···
 		return baycom_setmode(bc, hi.data.modename);
 
 	case HDLCDRVCTL_MODELIST:
-		strncpy(hi.data.modename, "intclk,extclk,intmodem,extmodem,divider=x",
+		strscpy_pad(hi.data.modename, "intclk,extclk,intmodem,extmodem,divider=x",
 			sizeof(hi.data.modename));
 		break;