Set zvol discard_granularity to the volblocksize.

Currently, zvols have a discard granularity set to 0, which suggests to
the upper layer that discard requests of arbirarily small size and
alignment can be made efficiently.

In practice however, ZFS does not handle unaligned discard requests
efficiently: indeed, it is unable to free a part of a block. It will
write zeros to the specified range instead, which is both useless and
inefficient (see dnode_free_range).

With this patch, zvol block devices expose volblocksize as their discard
granularity, so the upper layer is aware that it's not supposed to send
discard requests smaller than volblocksize.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #862

config/kernel-discard-granularity.m4

@@ -0,0 +1,20 @@
+dnl #
+dnl # 2.6.33 API change
+dnl # Discard granularity and alignment restrictions may now be set.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_DISCARD_GRANULARITY], [
+	AC_MSG_CHECKING([whether ql->discard_granularity is available])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/blkdev.h>
+	],[
+		struct queue_limits ql __attribute__ ((unused));
+
+		ql.discard_granularity = 0;
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_DISCARD_GRANULARITY, 1,
+		          [ql->discard_granularity is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -39,6 +39,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_GET_GENDISK
 	ZFS_AC_KERNEL_RQ_IS_SYNC
 	ZFS_AC_KERNEL_RQ_FOR_EACH_SEGMENT
+	ZFS_AC_KERNEL_DISCARD_GRANULARITY
 	ZFS_AC_KERNEL_CONST_XATTR_HANDLER
 	ZFS_AC_KERNEL_XATTR_HANDLER_GET
 	ZFS_AC_KERNEL_XATTR_HANDLER_SET