config: fix ZFS_LINUX_TEST_RESULT_SYMBOL with --enable-linux-builtin

The tiniest typo in dd2a46b (openzfs#17106) broke it, by setting the wrong
var with the test var, resulting in it always producing "no".

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <[email protected]>

Author: robn

config/kernel.m4

@@ -897,7 +897,7 @@ AC_DEFUN([ZFS_LINUX_TEST_RESULT_SYMBOL], [
 					eval "$cachevar=no"
 				])
 			], [
-				eval "$cacheval=yes"
+				eval "$cachevar=yes"
 			])
 		])
 	])