Remove the bio_empty_barrier() check.

To determine whether the kernel is capable of handling empty barrier
BIOs, we check for the presence of the bio_empty_barrier() macro,
which was introduced in 2.6.24. If this macro is defined, then we can
flush disk vdevs; if it isn't, then flushing is disabled.

Unfortunately, the bio_empty_barrier() macro was removed in 2.6.37,
even though the kernel is still capable of handling empty barrier BIOs.

As a result, flushing is effectively disabled on kernels >= 2.6.37,
meaning that starting from this kernel version, zfs doesn't use
barriers to guarantee on-disk data consistency. This is quite bad and
can lead to potential data corruption on power failures.

This patch fixes the issue by removing the configure check for
bio_empty_barrier(), as we don't support kernels <= 2.6.24 anymore.

Thanks to Richard Kojedzinszky for catching this nasty bug.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1318

config/kernel-bio-empty-barrier.m4

@@ -1,19 +0,0 @@
-dnl #
-dnl # 2.6.24 API change
-dnl # Empty write barriers are now supported and we should use them.
-dnl #
-AC_DEFUN([ZFS_AC_KERNEL_BIO_EMPTY_BARRIER], [
-	AC_MSG_CHECKING([whether bio_empty_barrier() is defined])
-	ZFS_LINUX_TRY_COMPILE([
-		#include <linux/bio.h>
-	],[
-		struct bio bio;
-		(void)bio_empty_barrier(&bio);
-	],[
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_BIO_EMPTY_BARRIER, 1,
-		          [bio_empy_barrier() is defined])
-	],[
-		AC_MSG_RESULT(no)
-	])
-])

config/kernel.m4

@@ -16,7 +16,6 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_INVALIDATE_BDEV_ARGS
 	ZFS_AC_KERNEL_BDEV_LOGICAL_BLOCK_SIZE
 	ZFS_AC_KERNEL_BDEV_PHYSICAL_BLOCK_SIZE
-	ZFS_AC_KERNEL_BIO_EMPTY_BARRIER
 	ZFS_AC_KERNEL_BIO_FAILFAST
 	ZFS_AC_KERNEL_BIO_FAILFAST_DTD
 	ZFS_AC_KERNEL_REQ_FAILFAST_MASK