Add support for DISCARD to ZVOLs.

DISCARD (REQ_DISCARD, BLKDISCARD) is useful for thin provisioning.
It allows ZVOL clients to discard (unmap, trim) block ranges from
a ZVOL, thus optimizing disk space usage by allowing a ZVOL to
shrink instead of just grow.

We can't use zfs_space() or zfs_freesp() here, since these functions
only work on regular files, not volumes. Fortunately we can use the
low-level function dmu_free_long_range() which does exactly what we
want.

Currently the discard operation is not added to the log. That's not
a big deal since losing discard requests cannot result in data
corruption. It would however result in disk space usage higher than
it should be. Thus adding log support to zvol_discard() is probably
a good idea for a future improvement.

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

config/kernel-blk-queue-discard.m4

@@ -0,0 +1,22 @@
+dnl #
+dnl # 2.6.32 API change
+dnl # Discard requests were moved to the normal I/O path.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_DISCARD], [
+	AC_MSG_CHECKING([whether blk_queue_discard() is available])
+	tmp_flags="$EXTRA_KCFLAGS"
+	EXTRA_KCFLAGS="-Wno-unused-but-set-variable"
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/blkdev.h>
+	],[
+		struct request_queue *q = NULL;
+		(void) blk_queue_discard(q);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_BLK_QUEUE_DISCARD, 1,
+		          [blk_queue_discard() is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+	EXTRA_KCFLAGS="$tmp_flags"
+])

config/kernel.m4

@@ -27,6 +27,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_BLK_QUEUE_PHYSICAL_BLOCK_SIZE
 	ZFS_AC_KERNEL_BLK_QUEUE_IO_OPT
 	ZFS_AC_KERNEL_BLK_QUEUE_NONROT
+	ZFS_AC_KERNEL_BLK_QUEUE_DISCARD
 	ZFS_AC_KERNEL_BLK_FETCH_REQUEST
 	ZFS_AC_KERNEL_BLK_REQUEUE_REQUEST
 	ZFS_AC_KERNEL_BLK_RQ_BYTES