Set HAVE_FS_STRUCT_SPINLOCK correctly when CONFIG_FRAME_WARN==1024

If kernel lock debugging is enabled, the fs_struct structure exceeds the
typical 1024 byte limit of CONFIG_FRAME_WARN and isn't enabled when it
otherwise should be.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes openzfs#440

config/spl-build.m4

@@ -1033,7 +1033,7 @@ AC_DEFUN([SPL_AC_FS_STRUCT_SPINLOCK], [
 		#include <linux/sched.h>
 		#include <linux/fs_struct.h>
 	],[
-		struct fs_struct fs;
+		static struct fs_struct fs;
 		spin_lock_init(&fs.lock);
 	],[
 		AC_MSG_RESULT(yes)