ddf tests: fix get_rootdev

Getting the major number from the hex device number should take
all-but-the-last-two digits, rather than just the first two digits.

Signed-off-by: NeilBrown <neilb@suse.de>

tests/env-ddf-template

@@ -4,7 +4,7 @@ sha1_sum() {
 
 get_rootdev() {
     local dev=$(stat -c %D /)
-    local maj=$(expr $dev : '\(..\)')
+    local maj=$(expr $dev : '\(..*\)..')
     local min=${dev#$maj}
     local bd=/dev/$(basename $(readlink /sys/dev/block/$((0x$maj)):$((0x$min))))
     [ -b $bd ] || exit 1