selftests: ublk: move creating UBLK_TMP into _prep_test()

test may exit early because of missing program or not having required
feature before calling _prep_test(), then $UBLK_TMP isn't cleaned.

Fix it by moving creating $UBLK_TMP into _prep_test(), any resources
created since _prep_test() will be cleaned by _cleanup_test().

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-14-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/test_common.sh

@@ -114,6 +114,7 @@ _prep_test() {
 	local type=$1
 	shift 1
 	modprobe ublk_drv > /dev/null 2>&1
+	UBLK_TMP=$(mktemp ublk_test_XXXXX)
 	[ "$UBLK_TEST_QUIET" -eq 0 ] && echo "ublk $type: $*"
 }
 
@@ -338,7 +339,6 @@ _ublk_test_top_dir()
 	cd "$(dirname "$0")" && pwd
 }
 
-UBLK_TMP=$(mktemp ublk_test_XXXXX)
 UBLK_PROG=$(_ublk_test_top_dir)/kublk
 UBLK_TEST_QUIET=1
 UBLK_TEST_SHOW_RESULT=1