iscsi-target; Enforce 1024 byte maximum for CHAP_C key value

This patch adds a check in chap_server_compute_md5() to enforce a
1024 byte maximum for the CHAP_C key value following the requirement
in RFC-3720 Section 11.1.4:

   "..., C and R are large-binary-values and their binary length (not
   the length of the character string that represents them in encoded
   form) MUST not exceed 1024 bytes."

Reported-by: rahul.rane <rahul.rane@calsoftinc.com>
Tested-by: rahul.rane <rahul.rane@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>