io_uring: fix crash with IORING_SETUP_NO_MMAP and invalid SQ ring address

If we specify a valid CQ ring address but an invalid SQ ring address,
we'll correctly spot this and free the allocated pages and clear them
to NULL. However, we don't clear the ring page count, and hence will
attempt to free the pages again. We've already cleared the address of
the page array when freeing them, but we don't check for that. This
causes the following crash:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Oops [#1]
Modules linked in:
CPU: 0 PID: 20 Comm: kworker/u2:1 Not tainted 6.6.0-rc5-dirty #56
Hardware name: ucbbar,riscvemu-bare (DT)
Workqueue: events_unbound io_ring_exit_work
epc : io_pages_free+0x2a/0x58
 ra : io_rings_free+0x3a/0x50
 epc : ffffffff808811a2 ra : ffffffff80881406 sp : ffff8f80000c3cd0
 status: 0000000200000121 badaddr: 0000000000000000 cause: 000000000000000d
 [<ffffffff808811a2>] io_pages_free+0x2a/0x58
 [<ffffffff80881406>] io_rings_free+0x3a/0x50
 [<ffffffff80882176>] io_ring_exit_work+0x37e/0x424
 [<ffffffff80027234>] process_one_work+0x10c/0x1f4
 [<ffffffff8002756e>] worker_thread+0x252/0x31c
 [<ffffffff8002f5e4>] kthread+0xc4/0xe0
 [<ffffffff8000332a>] ret_from_fork+0xa/0x1c

Check for a NULL array in io_pages_free(), but also clear the page counts
when we free them to be on the safer side.

Reported-by:  <rtm@csail.mit.edu>
Fixes: 03d89a2d ("io_uring: support for user allocated memory for rings/sqes")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>