smb: Work around Clang __bdos() type confusion

Recent versions of Clang gets confused about the possible size of the
"user" allocation, and CONFIG_FORTIFY_SOURCE ends up emitting a
warning[1]:

repro.c:126:4: warning: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  126 |                         __write_overflow_field(p_size_field, size);
      |                         ^

for this memset():

        int len;
        __le16 *user;
	...
        len = ses->user_name ? strlen(ses->user_name) : 0;
        user = kmalloc(2 + (len * 2), GFP_KERNEL);
	...
	if (len) {
		...
	} else {
		memset(user, '\0', 2);
	}

While Clang works on this bug[2], switch to using a direct assignment,
which avoids memset() entirely which both simplifies the code and silences
the false positive warning. (Making "len" size_t also silences the
warning, but the direct assignment seems better.)

Reported-by: Nathan Chancellor <[email protected]>
Closes: ClangBuiltLinux/linux#1966 [1]
Link: llvm/llvm-project#77813 [2]
Cc: Steve French <[email protected]>
Cc: Paulo Alcantara <[email protected]>
Cc: Ronnie Sahlberg <[email protected]>
Cc: Shyam Prasad N <[email protected]>
Cc: Tom Talpey <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Steve French <[email protected]>

fs/smb/client/cifsencrypt.c

@@ -572,7 +572,7 @@ static int calc_ntlmv2_hash(struct cifs_ses *ses, char *ntlmv2_hash,
 		len = cifs_strtoUTF16(user, ses->user_name, len, nls_cp);
 		UniStrupr(user);
 	} else {
-		memset(user, '\0', 2);
+		*(u16 *)user = 0;
 	}
 
 	rc = crypto_shash_update(ses->server->secmech.hmacmd5,