libspl: staticify buf and pagesize, rename aok to libspl_assert_ok

Exporting names this short can easily cause nasty collisions with user code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12050

lib/libspl/assert.c

@@ -25,7 +25,7 @@
 
 #include <assert.h>
 
-int aok = 0;
+int libspl_assert_ok = 0;
 
 /* printf version of libspl_assert */
 void
@@ -39,7 +39,7 @@ libspl_assertf(const char *file, const char *func, int line,
 	fprintf(stderr, "\n");
 	fprintf(stderr, "ASSERT at %s:%d:%s()", file, line, func);
 	va_end(args);
-	if (aok) {
+	if (libspl_assert_ok) {
 		return;
 	}
 	abort();