LSM: Separate idea of "major" LSM from "exclusive" LSM

In order to both support old "security=" Legacy Major LSM selection, and
handling real exclusivity, this creates LSM_FLAG_EXCLUSIVE and updates
the selection logic to handle them.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>

include/linux/lsm_hooks.h

@@ -2043,6 +2043,7 @@ extern void security_add_hooks(struct security_hook_list *hooks, int count,
 				char *lsm);
 
 #define LSM_FLAG_LEGACY_MAJOR	BIT(0)
+#define LSM_FLAG_EXCLUSIVE	BIT(1)
 
 struct lsm_info {
 	const char *name;	/* Required. */