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

cmd/zdb/zdb.c

@@ -8496,7 +8496,7 @@ main(int argc, char **argv)
 			dump_opt[c] += verbose;
 	}
 
-	aok = (dump_opt['A'] == 1) || (dump_opt['A'] > 2);
+	libspl_assert_ok = (dump_opt['A'] == 1) || (dump_opt['A'] > 2);
 	zfs_recover = (dump_opt['A'] > 1);
 
 	argc -= optind;