iscsi-target: chap auth shouldn't match username with trailing garbage

In iSCSI negotiations with initiator CHAP enabled, usernames with
trailing garbage are permitted, because the string comparison only
checks the strlen of the configured username.

e.g. "usernameXXXXX" will be permitted to match "username".

Just check one more byte so the trailing null char is also matched.

Signed-off-by: Eric Seppanen <eric@purestorage.com>
Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>