xfs: check for deleted cursors when revalidating two btrees

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

xfs: check for deleted cursors when revalidating two btrees

The free space and inode btree repair functions will rebuild both btrees
at the same time, after which it needs to evaluate both btrees to
confirm that the corruptions are gone.

However, Jiaming Zhang ran syzbot and produced a crash in the second
xchk_allocbt call. His root-cause analysis is as follows (with minor
corrections):

In xrep_revalidate_allocbt(), xchk_allocbt() is called twice (first
for BNOBT, second for CNTBT). The cause of this issue is that the
first call nullified the cursor required by the second call.

Let's first enter xrep_revalidate_allocbt() via following call chain:

xfs_file_ioctl() ->
xfs_ioc_scrubv_metadata() ->
xfs_scrub_metadata() ->
`sc->ops->repair_eval(sc)` ->
xrep_revalidate_allocbt()

xchk_allocbt() is called twice in this function. In the first call:

/* Note that sc->sm->sm_type is XFS_SCRUB_TYPE_BNOPT now */
xchk_allocbt() ->
xchk_btree() ->
`bs->scrub_rec(bs, recp)` ->
xchk_allocbt_rec() ->
xchk_allocbt_xref() ->
xchk_allocbt_xref_other()

since sm_type is XFS_SCRUB_TYPE_BNOBT, pur is set to &sc->sa.cnt_cur.
Kernel called xfs_alloc_get_rec() and returned -EFSCORRUPTED. Call
chain:

xfs_alloc_get_rec() ->
xfs_btree_get_rec() ->
xfs_btree_check_block() ->
(XFS_IS_CORRUPT || XFS_TEST_ERROR), the former is false and the latter
is true, return -EFSCORRUPTED. This should be caused by
ioctl$XFS_IOC_ERROR_INJECTION I guess.

Back to xchk_allocbt_xref_other(), after receiving -EFSCORRUPTED from
xfs_alloc_get_rec(), kernel called xchk_should_check_xref(). In this
function, *curpp (points to sc->sa.cnt_cur) is nullified.

Back to xrep_revalidate_allocbt(), since sc->sa.cnt_cur has been
nullified, it then triggered null-ptr-deref via xchk_allocbt() (second
call) -> xchk_btree().

So. The bnobt revalidation failed on a cross-reference attempt, so we
deleted the cntbt cursor, and then crashed when we tried to revalidate
the cntbt. Therefore, check for a null cntbt cursor before that
revalidation, and mark the repair incomplete. Also we can ignore the
second tree entirely if the first tree was rebuilt but is already
corrupt.

Apply the same fix to xrep_revalidate_iallocbt because it has the same
problem.