CVE-2022-48733: btrfs: fix use-after-free after failure to create a snapshot

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix use-after-free after failure to create a snapshot

At ioctl.c:createsnapshot(), we allocate a pending snapshot structure and then attach it to the transaction's list of pending snapshots. After that we call btrfscommittransaction(), and if that returns an error we jump to 'fail' label, where we kfree() the pending snapshot structure. This can result in a later use-after-free of the pending snapshot:

1) We allocated the pending snapshot and added it to the transaction's list of pending snapshots;

2) We call btrfscommittransaction(), and it fails either at the first call to btrfsrundelayedrefs() or btrfsstartdirtyblockgroups(). In both cases, we don't abort the transaction and we release our transaction handle. We jump to the 'fail' label and free the pending snapshot structure. We return with the pending snapshot still in the transaction's list;

3) Another task commits the transaction. This time there's no error at all, and then during the transaction commit it accesses a pointer to the pending snapshot structure that the snapshot creation task has already freed, resulting in a user-after-free.

This issue could actually be detected by smatch, which produced the following warning:

fs/btrfs/ioctl.c:843 createsnapshot() warn: '&pendingsnapshot->list' not removed from list

So fix this by not having the snapshot creation ioctl directly add the pending snapshot to the transaction's list. Instead add the pending snapshot to the transaction handle, and then at btrfscommittransaction() we add the snapshot to the list only when we can guarantee that any error returned after that point will result in a transaction abort, in which case the ioctl code can safely free the pending snapshot and no one can access it anymore.