qemu-ga: execute fsfreeze-freeze in reverse order of mounts

Currently, fsfreeze-freeze may cause deadlock if a guest has loopback mounts
of image files in its disk; e.g.:

    # mount | grep ^/
    /dev/vda1 / type ext4 (rw,noatime,seclabel,data=ordered)
    /tmp/disk.img on /mnt type ext4 (rw,relatime,seclabel)

To avoid the deadlock, this freezes filesystems in reverse order of mounts.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
*fix up commit msg
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit e5d9adb)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -566,7 +566,7 @@ typedef struct FsMount {
     QTAILQ_ENTRY(FsMount) next;
 } FsMount;
 
-typedef QTAILQ_HEAD(, FsMount) FsMountList;
+typedef QTAILQ_HEAD(FsMountList, FsMount) FsMountList;
 
 static void free_fs_mount_list(FsMountList *mounts)
 {
@@ -728,7 +728,7 @@ int64_t qmp_guest_fsfreeze_freeze(Error **err)
     /* cannot risk guest agent blocking itself on a write in this state */
     ga_set_frozen(ga_state);
 
-    QTAILQ_FOREACH(mount, &mounts, next) {
+    QTAILQ_FOREACH_REVERSE(mount, &mounts, FsMountList, next) {
         fd = qemu_open(mount->dirname, O_RDONLY);
         if (fd == -1) {
             error_setg_errno(err, errno, "failed to open %s", mount->dirname);