staging: rtl8723bs: Allocate full pwep structure

The pwep allocation was always being allocated smaller than the true
structure size. Avoid this by always allocating the full structure.
Found with GCC 12 and -Warray-bounds:

../drivers/staging/rtl8723bs/os_dep/ioctl_linux.c: In function 'rtw_set_encryption':
../drivers/staging/rtl8723bs/os_dep/ioctl_linux.c:591:29: warning: array subscript 'struct ndis_802_11_wep[0]' is partly outside array bounds of 'void[25]' [-Warray-bounds]
591 | pwep->length = wep_total_len;
| ^~

Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Fabio Aiuto <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/rtl8723bs/os_dep/ioctl_linux.c

···
 		if (wep_key_len > 0) {
 			wep_key_len = wep_key_len <= 5 ? 5 : 13;
 			wep_total_len = wep_key_len + FIELD_OFFSET(struct ndis_802_11_wep, key_material);
-			pwep = kzalloc(wep_total_len, GFP_KERNEL);
+			/* Allocate a full structure to avoid potentially running off the end. */
+			pwep = kzalloc(sizeof(*pwep), GFP_KERNEL);
 			if (!pwep) {
 				ret = -ENOMEM;
 				goto exit;
···
 		if (wep_key_len > 0) {
 			wep_key_len = wep_key_len <= 5 ? 5 : 13;
 			wep_total_len = wep_key_len + FIELD_OFFSET(struct ndis_802_11_wep, key_material);
-			pwep = kzalloc(wep_total_len, GFP_KERNEL);
+			/* Allocate a full structure to avoid potentially running off the end. */
+			pwep = kzalloc(sizeof(*pwep), GFP_KERNEL);
 			if (!pwep)
 				goto exit;