Linux kernel XFS: recovery of unlinked inodes can leak, loop, or use freed memory

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

xfs: fix cursor and pointer handling when recovering iunlink buckets

LOLLM pointed out a bug in xlog_recover_iunlink_bucket:

1. We don't null out prev_ip after releasing it, which can lead to UAF problems if the inodegc flush call in the loop fails.

at which point I noticed even more bugs:

1. If the inodegc flush inside the loop fails, we also leak @ip.

2. We set prev_agino to agino having already advanced agino, which results in inodes with i_prev_unlinked set to itself.

3. If we exit the bottom of the loop with prev_ip set, then prev_ip aliases ip and we also set its i_prev_unlinked to itself.

Bugs 3 and 4 introduce loops into the unlinked list, though these loops don't surface because we immediately flush each unlinked inode after loading it.

Fix all of these issues.