grub-fs-tester: Fix losetup race

If something else on the system is using loopback devices, then the
device that's free at the call to `losetup -f` may not be free in the
following call to try to use it. Instead, find and use the first free
loopback device in a single call to losetup.

Signed-off-by: Will Thompson <wjt@endlessm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

tests/util/grub-fs-tester.in

@@ -616,9 +616,8 @@ for LOGSECSIZE in $(range "$MINLOGSECSIZE" "$MAXLOGSECSIZE" 1); do
 		    mkdir -p "$MNTPOINTRO"
 		    for i in $(range 0 $((NDEVICES-1)) 1); do
 			dd if=/dev/zero of="$FSIMAGEP${i}.img" count=1 bs=1 seek=$((DISKSIZE-1)) &> /dev/null
-			LODEVICE="$(losetup -f)"
+			LODEVICE=$(losetup --find --show "$FSIMAGEP${i}.img")
 			LODEVICES="$LODEVICES $LODEVICE"
-			losetup "$LODEVICE" "$FSIMAGEP${i}.img"
 			if test "$i" = 0; then
 			    MOUNTDEVICE="$LODEVICE"
 			fi