Shrink stack space usage in cifs_construct_tcon

We were reserving MAX_USERNAME (now 256) on stack for
something which only needs to fit about 24 bytes ie
string krb50x +  printf version of uid
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsglob.h

@@ -274,7 +274,8 @@ struct cifsSesInfo {
 	int capabilities;
 	char serverName[SERVER_NAME_LEN_WITH_NULL * 2];	/* BB make bigger for
 				TCP names - will ipv6 and sctp addresses fit? */
-	char *user_name;
+	char *user_name;	/* must not be null except during init of sess
+				   and after mount option parsing we fill it */
 	char *domainName;
 	char *password;
 	struct session_key auth_key;

fs/cifs/connect.c

@@ -3258,7 +3258,9 @@ cifs_construct_tcon(struct cifs_sb_info *cifs_sb, uid_t fsuid)
 	struct cifsSesInfo *ses;
 	struct cifsTconInfo *tcon = NULL;
 	struct smb_vol *vol_info;
-	char username[MAX_USERNAME_SIZE + 1];
+	char username[28]; /* big enough for "krb50x" + hex of ULONG_MAX 6+16 */
+			   /* We used to have this as MAX_USERNAME which is   */
+			   /* way too big now (256 instead of 32) */
 
 	vol_info = kzalloc(sizeof(*vol_info), GFP_KERNEL);
 	if (vol_info == NULL) {