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

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 <abuchbinder@google.com>
Reviewed-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>