fix incorrect detecting readv/writev.

Branch b1_6
b=17918
i=green

lustre/autoconf/lustre-core.m4

@@ -993,7 +993,7 @@ AC_DEFUN([LC_FILE_WRITEV],
 LB_LINUX_TRY_COMPILE([
         #include <linux/fs.h>
 ],[
-        struct file_operations *fops;
+        struct file_operations *fops = NULL;
         fops->writev = NULL;
 ],[
         AC_MSG_RESULT(yes)
@@ -1011,7 +1011,7 @@ AC_DEFUN([LC_FILE_READV],
 LB_LINUX_TRY_COMPILE([
         #include <linux/fs.h>
 ],[
-        struct file_operations *fops;
+        struct file_operations *fops = NULL;
         fops->readv = NULL;
 ],[
         AC_MSG_RESULT(yes)