btrfs-progs: test/fsck/021: Cleanup custom check by overriding check_image

Signed-off-by: Qu Wenruo <wqu@suse.com>
[ update comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

tests/fsck-tests/021-partially-dropped-snapshot-case/test.sh

@@ -1,18 +1,22 @@
 #!/bin/bash
-# confirm whether btrfsck supports to check a partially dropped snapshot
+# confirm whether 'btrfs check' supports check ing of a partially dropped
+# snapshot
 
 source "$TOP/tests/common"
 
 check_prereq btrfs
 
-for img in *.img
-do
-	image=$(extract_image "$img")
+check_image()
+{
+	local image
+
+	image=$1
 	run_check_stdout "$TOP/btrfs" check "$image" 2>&1 |
 		grep -q "Errors found in extent allocation tree or chunk allocation"
 	if [ $? -eq 0 ]; then
 		rm -f "$image"
 		_fail "unexpected error occurred when checking $img"
 	fi
-	rm -f "$image"
-done
+}
+
+check_all_images