target/file: Update hw_max_sectors based on current block_size

This patch allows FILEIO to update hw_max_sectors based on the current
max_bytes_per_io.  This is required because vfs_[writev,readv]() can accept
a maximum of 2048 iovecs per call, so the enforced hw_max_sectors really
needs to be calculated based on block_size.

This addresses a >= v3.5 bug where block_size=512 was rejecting > 1M
sized I/O requests, because FD_MAX_SECTORS was hardcoded to 2048 for
the block_size=4096 case.

(v2: Use max_bytes_per_io instead of ->update_hw_max_sectors)

Reported-by: Henrik Goldman <hg@x-formation.com>
Cc: <stable@vger.kernel.org> #3.5+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
index 207b340..d06de84 100644
--- a/drivers/target/target_core_device.c
+++ b/drivers/target/target_core_device.c
@@ -1106,6 +1106,11 @@
 	dev->dev_attrib.block_size = block_size;
 	pr_debug("dev[%p]: SE Device block_size changed to %u\n",
 			dev, block_size);
+
+	if (dev->dev_attrib.max_bytes_per_io)
+		dev->dev_attrib.hw_max_sectors =
+			dev->dev_attrib.max_bytes_per_io / block_size;
+
 	return 0;
 }