[CIFS] update limit for snprintf in cifs_construct_tcon

In 34c87901 "Shrink stack space usage in cifs_construct_tcon" we
change the size of the username name buffer from MAX_USERNAME_SIZE
(256) to 28.  This call to snprintf() needs to be updated as well.
Reported by Dan Carpenter.

Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>