ksmbd: avoid field overflow warning

clang warns about a possible field overflow in a memcpy:

In file included from fs/smb/server/smb_common.c:7:
include/linux/fortify-string.h:583:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
                        __write_overflow_field(p_size_field, size);

It appears to interpret the "&out[baselen + 4]" as referring to a single
byte of the character array, while the equivalen "out + baselen + 4" is
seen as an offset into the array.

I don't see that kind of warning elsewhere, so just go with the simple
rework.

Fixes: e2f3448 ("cifsd: add server-side procedures for SMB3")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/smb_common.c

@@ -536,7 +536,7 @@ int ksmbd_extract_shortname(struct ksmbd_conn *conn, const char *longname,
 	out[baselen + 3] = PERIOD;
 
 	if (dot_present)
-		memcpy(&out[baselen + 4], extension, 4);
+		memcpy(out + baselen + 4, extension, 4);
 	else
 		out[baselen + 4] = '\0';
 	smbConvertToUTF16((__le16 *)shortname, out, PATH_MAX,