libxfs: don't UAF a requeued EFI

libxfs: don't UAF a requeued EFI

In the kernel, commit 8ebbf262d4684 ("xfs: don't block in busy flushing
when freeing extents") changed the allocator behavior such that AGFL
fixing can return -EAGAIN in response to detection of a deadlock with
the transaction busy extent list.  If this happens, we're supposed to
requeue the EFI so that we can roll the transaction and try the item
again.

If a requeue happens, we should not free the xefi pointer in
xfs_extent_free_finish_item or else the retry will walk off a dangling
pointer.  There is no extent busy list in userspace so this should
never happen, but let's fix the logic bomb anyway.

We should have ported kernel commit 0853b5de42b47 ("xfs: allow extent
free intents to be retried") to userspace, but neither Carlos nor I
noticed this fine detail. :(

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanbabu@kernel.org>