selftests/bpf: Update ima test helper's losetup commands

Update the commands to use the bare minimum options so that it works
in busybox environments.

Fixes: 34b82d3 ("bpf: Add a selftest for bpf_ima_inode_hash")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: KP Singh <kpsingh@google.com>

Author: sinkap

tools/testing/selftests/bpf/ima_setup.sh

@@ -3,6 +3,7 @@
 
 set -e
 set -u
+set -o pipefail
 
 IMA_POLICY_FILE="/sys/kernel/security/ima/policy"
 TEST_BINARY="/bin/true"
@@ -23,9 +24,10 @@ setup()
 
         dd if=/dev/zero of="${mount_img}" bs=1M count=10
 
-        local loop_device="$(losetup --find --show ${mount_img})"
+        losetup -f "${mount_img}"
+        local loop_device=$(losetup -a | grep ${mount_img:?} | cut -d ":" -f1)
 
-        mkfs.ext4 "${loop_device}"
+        mkfs.ext4 "${loop_device:?}"
         mount "${loop_device}" "${mount_dir}"
 
         cp "${TEST_BINARY}" "${mount_dir}"
@@ -38,7 +40,8 @@ cleanup() {
         local mount_img="${tmp_dir}/test.img"
         local mount_dir="${tmp_dir}/mnt"
 
-        local loop_devices=$(losetup -j ${mount_img} -O NAME --noheadings)
+        local loop_devices=$(losetup -a | grep ${mount_img:?} | cut -d ":" -f1)
+
         for loop_dev in "${loop_devices}"; do
                 losetup -d $loop_dev
         done