xfs: Fix a CIL UAF by getting get rid of the iclog callback lock

The iclog callback chain has it's own lock. That was added way back
in 2008 by myself to alleviate severe lock contention on the
icloglock in commit 114d23aa ("[XFS] Per iclog callback chain
lock"). This was long before delayed logging took the icloglock out
of the hot transaction commit path and removed all contention on it.
Hence the separate ic_callback_lock doesn't serve any scalability
purpose anymore, and hasn't for close on a decade.

Further, we only attach callbacks to iclogs in one place where we
are already taking the icloglock soon after attaching the callbacks.
We also have to drop the icloglock to run callbacks and grab it
immediately afterwards again. So given that the icloglock is no
longer hot, making it cover callbacks again doesn't really change
the locking patterns very much at all.

We also need to extend the icloglock to cover callback addition to
fix a zero-day UAF in the CIL push code. This occurs when shutdown
races with xlog_cil_push_work() and the shutdown runs the callbacks
before the push releases the iclog. This results in the CIL context
structure attached to the iclog being freed by the callback before
the CIL push has finished referencing it, leading to UAF bugs.

Hence, to avoid this UAF, we need the callback attachment to be
atomic with post processing of the commit iclog and references to
the structures being attached to the iclog. This requires holding
the icloglock as that's the only way to serialise iclog state
against a shutdown in progress.

The result is we need to be using the icloglock to protect the
callback list addition and removal and serialise them with shutdown.
That makes the ic_callback_lock redundant and so it can be removed.

Fixes: 71e330b5 ("xfs: Introduce delayed logging core code")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>