block/042: add description and fix shellcheck warnings

Based on the commit message of the commit 57edc10 ("blktests: test
direct io offsets"), add description comment to the test case block/042.
Also, fix several shellcheck warnings.

Signed-off-by: Shin'ichiro Kawasaki <[email protected]>

Author: kawasaki

tests/block/042

@@ -1,6 +1,9 @@
 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 # Copyright 2025 Keith Busch <[email protected]>
+#
+# Tests various direct IO memory and length alignments against the device's
+# queue limits reported from sysfs.
 
 . tests/block/rc
 
@@ -20,7 +23,9 @@ test_device() {
 	sys_logical_block_size=$(cat "${TEST_DEV_SYSFS}/queue/logical_block_size")
 	sys_max_sectors_kb=$(cat "${TEST_DEV_SYSFS}/queue/max_sectors_kb")
 
-	if ! src/dio-offsets ${TEST_DEV} $sys_max_segments $sys_max_sectors_kb $sys_dma_alignment $sys_virt_boundary_mask $sys_logical_block_size; then
+	if ! src/dio-offsets "${TEST_DEV}" "$sys_max_segments" \
+	     "$sys_max_sectors_kb" "$sys_dma_alignment" \
+	     "$sys_virt_boundary_mask" "$sys_logical_block_size"; then
 		echo "src/dio-offsets failed"
 	fi