btrfs-progs: btrfstune: fix a bug which makes unfinished fsid change unrecoverable

btrfs-progs: btrfstune: fix a bug which makes unfinished fsid change unrecoverable

This bug is found by making break point after change_fsid_prepare() and
then kill the unfinished change, then try to restore the unfinished fsid
change.

If fsid change is canceled, open_ctree will still fail even with
IGNORE_FSID_MIMATCH open ctree flag, since it can't find device with
mismatched fsid, making it unable to restoring.

Now add ignore_fsid_mismatch judgment in btrfs_find_device() to fix the
bug and allow later restore to work as expected.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>