btrfs-progs: Fix a use-after-free in the volumes code.

When a struct btrfs_fs_devices was being torn down by
btrfs_close_devices(), there was an invalidated pointer in the global
list fs_uuids which still pointed to it; if a device was closed and
then reopened (which btrfs-convert does), freed memory would be
accessed.

This was found using ThreadSanitizer (pretty much doing what
AddressSanitizer would, but not exiting after the first failure).
To reproduce, build with -fsanitize=thread and run 'make test'.
Representative output is below.

This change makes the current tests TSan-clean.

WARNING: ThreadSanitizer: heap-use-after-free (pid=29161)
  Read of size 8 at 0x7d180000eee0 by main thread:
    #0 memcmp ??:0
    #1 find_fsid .../volumes.c:81
    #2 device_list_add .../volumes.c:95
    #3 btrfs_scan_one_device .../volumes.c:259
    #4 btrfs_scan_fs_devices .../disk-io.c:1002
    #5 __open_ctree_fd .../disk-io.c:1090
    #6 open_ctree_fd .../disk-io.c:1191
    #7 do_convert .../btrfs-convert.c:2317
    #8 main .../btrfs-convert.c:2745

  Previous write of size 8 at 0x7d180000eee0 by main thread:
    #0 free ??:0
    #1 btrfs_close_devices .../volumes.c:191
    #2 close_ctree .../disk-io.c:1401
    #3 do_convert .../btrfs-convert.c:2300
    #4 main .../btrfs-convert.c:2745

  Location is heap block of size 96 at 0x7d180000eee0 allocated by main thread:
    #0 calloc ??:0 (exe+0x00000002acc6)
    #1 device_list_add .../volumes.c:97
    #2 btrfs_scan_one_device .../volumes.c:259
    #3 btrfs_scan_fs_devices .../disk-io.c:1002
    #4 __open_ctree_fd .../disk-io.c:1090
    #5 open_ctree_fd .../disk-io.c:1191
    #6 do_convert .../btrfs-convert.c:2256
    #7 main .../btrfs-convert.c:2745

Signed-off-by: Adam Buchbinder <[email protected]>
Reviewed-by: Satoru Takeuchi <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adambuchbinder

volumes.c

@@ -188,6 +188,7 @@ int btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 		goto again;
 	}
 
+	list_del(&fs_devices->list);
 	free(fs_devices);
 	return 0;
 }

volumes.h

@@ -67,7 +67,7 @@ struct btrfs_device {
 struct btrfs_fs_devices {
 	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
 
-	/* the device with this id has the most recent coyp of the super */
+	/* the device with this id has the most recent copy of the super */
 	u64 latest_devid;
 	u64 latest_trans;
 	u64 lowest_devid;