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

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

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <[email protected]>
Reviewed by: Attila Fülöp <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #17236

Author: robn

Diff for: config/kernel.m4

@@ -899,7 +899,7 @@ AC_DEFUN([ZFS_LINUX_TEST_RESULT_SYMBOL], [
 					eval "$cachevar=no"
 				])
 			], [
-				eval "$cacheval=yes"
+				eval "$cachevar=yes"
 			])
 		])
 	])