Fix HAVE_FS_STRUCT_SPINLOCK check for gcc-4.1.2

Older versions of gcc (gcc-4.1.2) will treat an 'incompatible
pointer type' as a warning instead of an error.  This results
in HAVE_FS_STRUCT_SPINLOCK being defined incorrectly.  This
failure mode was observed when using a RHEL6 2.6.32 based kernel
under RHEL5.5 which contains the old version of gcc.  To resolve
the issue the warning is explicitly promoted to an error.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

config/spl-build.m4

@@ -1598,6 +1598,8 @@ dnl # a spinlock_t to improve the fastpath performance.
 dnl #
 AC_DEFUN([SPL_AC_FS_STRUCT_SPINLOCK], [
 	AC_MSG_CHECKING([whether struct fs_struct uses spinlock_t])
+	tmp_flags="$EXTRA_KCFLAGS"
+	EXTRA_KCFLAGS="-Werror"
 	SPL_LINUX_TRY_COMPILE([
 		#include <linux/sched.h>
 		#include <linux/fs_struct.h>
@@ -1611,6 +1613,7 @@ AC_DEFUN([SPL_AC_FS_STRUCT_SPINLOCK], [
 	],[
 		AC_MSG_RESULT(no)
 	])
+	EXTRA_KCFLAGS="$tmp_flags"
 ])
 
 dnl #