btrfs-progs: image: Fix data races when reporting progress

btrfs-progs: image: Fix data races when reporting progress

Making the code data-race safe requires that reads *and* writes
happen under a mutex lock, if any of the access are writes. See
Dmitri Vyukov, "Benign data races: what could possibly go wrong?"
for more details.

The fix here was to put most of the main loop of restore_worker
under a mutex lock.

This race was detected using fsck-tests/012-leaf-corruption.

==================
WARNING: ThreadSanitizer: data race
  Write of size 4 by main thread:
    #0 add_cluster btrfs-progs/image/main.c:1931
    #1 restore_metadump btrfs-progs/image/main.c:2566
    #2 main btrfs-progs/image/main.c:2859

  Previous read of size 4 by thread T6:
    #0 restore_worker btrfs-progs/image/main.c:1720

  Location is stack of main thread.

  Thread T6 (running) created by main thread at:
    #0 pthread_create <null>
    #1 mdrestore_init btrfs-progs/image/main.c:1868
    #2 restore_metadump btrfs-progs/image/main.c:2534
    #3 main btrfs-progs/image/main.c:2859

SUMMARY: ThreadSanitizer: data race btrfs-progs/image/main.c:1931 in
add_cluster

Signed-off-by: Adam Buchbinder <abuchbinder@google.com>
Signed-off-by: David Sterba <dsterba@suse.com>