Fix vitastor-disk resize to smaller size clearing all data O_o, add a test for it

tests/test_rm.sh

@@ -10,4 +10,18 @@ $ETCDCTL get --prefix '/vitastor/pg/state'
 
 $VITASTOR_CLI rm-data --pool 1 --inode 1
 
+# test resize with partial data removal
+
+$VITASTOR_CLI create -s 128M testimg
+
+$VITASTOR_FIO -bs=4M -direct=1 -iodepth=1 -end_fsync=1 -fsync=1 -rw=write -image=testimg
+
+$VITASTOR_CLI modify --resize 64M testimg
+
+$VITASTOR_CLI dd iimg=testimg of=./testdata/bin/testimg.bin
+
+sz=$(stat -c %s ./testdata/bin/testimg.bin)
+
+[[ "$sz" = 67108864 ]] || format_error "File size is $sz bytes after resize to 64 M"
+
 format_green OK