scsi: target: compare full CHAP_A Algorithm strings

RFC 2307 states:

  For CHAP [RFC1994], in the first step, the initiator MUST send:

      CHAP_A=<A1,A2...>

   Where A1,A2... are proposed algorithms, in order of preference.
...
   For the Algorithm, as stated in [RFC1994], one value is required to
   be implemented:

       5     (CHAP with MD5)

LIO currently checks for this value by only comparing a single byte in
the tokenized Algorithm string, which means that any value starting with
a '5' (e.g. "55") is interpreted as "CHAP with MD5". Fix this by
comparing the entire tokenized string.
Reviewed-by: NLee Duncan <lduncan@suse.com>
Reviewed-by: NMike Christie <mchristi@redhat.com>
Signed-off-by: NDavid Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20190912095547.22427-2-ddiss@suse.deSigned-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/target/iscsi/iscsi_target_auth.c

@@ -70,7 +70,7 @@ static int chap_check_algorithm(const char *a_str)
 		if (!token)
 			goto out;
 
-		if (!strncmp(token, "5", 1)) {
+		if (!strcmp(token, "5")) {
 			pr_debug("Selected MD5 Algorithm\n");
 			kfree(orig);
 			return CHAP_DIGEST_MD5;