iscsi-scstd: Share ARRAY_SIZE for userspace dispatch bounds

Coccinelle misc/array_size.cocci reports:

  iscsi-scst/usr/iscsi_adm.c:595:33-34: WARNING: Use ARRAY_SIZE

Move the existing iSNS ARRAY_SIZE helper to misc.h and use it for the
user-command dispatch table in iscsi-scst-adm. Both users already include
that header, so they can share the definition.

Preserve the size_t comparison, including rejection of negative operation
values, and the unsupported-operation path. The table index is used
only after the bounds check succeeds.

iscsi-scst/usr/iscsi_adm.c

@@ -592,8 +592,8 @@ static int user_handle(int op, u32 set, u32 tid, char *params)
 	if (err)
 		goto out;
 
-	if ((op >= sizeof(user_handle_fn)/sizeof(user_handle_fn[0])) ||
-	    ((fn = user_handle_fn[op]) == NULL)) {
+	fn = op < ARRAY_SIZE(user_handle_fn) ? user_handle_fn[op] : NULL;
+	if (!fn) {
 		fprintf(stderr, "Unsupported\n");
 		goto out;
 	}

iscsi-scst/usr/isns.c

@@ -34,7 +34,6 @@
 #include "isns_proto.h"
 #include "misc.h"
 
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 #define BUFSIZE (1 << 18)
 
 struct isns_io {

iscsi-scst/usr/misc.h

@@ -16,6 +16,8 @@
 #ifndef MISC_H
 #define MISC_H
 
+#define ARRAY_SIZE(array) (sizeof(array) / sizeof((array)[0]))
+
 struct __qelem {
 	struct __qelem *q_forw;
 	struct __qelem *q_back;