btrfs-progs: tests: fix misc/005-long-device-name-for-ssd

We use a device mapper device on top of a loop device, the change in
rotational status does not always propagate if change it at the loop
device sysfs node.

Signed-off-by: David Sterba <dsterba@suse.com>

tests/mkfs-tests/005-long-device-name-for-ssd/test.sh

@@ -21,7 +21,8 @@ run_check truncate -s2g img
 loopdev=`run_check_stdout $SUDO_HELPER losetup --find --show img`
 run_check $SUDO_HELPER dmsetup create $dmname --table "0 1048576 linear $loopdev 0"
 
-base=`basename "$loopdev"`
+dmbase=`readlink -f $dmdev`
+base=`basename "$dmbase"`
 rot=/sys/class/block/$base/queue/rotational
 
 # switch rotational