md: account io_acct_set usage with active_io

io_acct_set was enabled for raid0/raid5 io accounting. bios that contain
md_io_acct are allocated in the i/o path. There isn't a good method to
monitor if these bios are all finished and freed. In the takeover process,
io_acct_set (which is used for bios with md_io_acct) need to be freed.
However, if some bios finish after io_acct_set is freed, it may trigger
the following panic:

[ 6973.767999] RIP: 0010:mempool_free+0x52/0x80
[ 6973.786098] Call Trace:
[ 6973.786549]  md_end_io_acct+0x31/0x40
[ 6973.787227]  blk_update_request+0x224/0x380
[ 6973.787994]  blk_mq_end_request+0x1a/0x130
[ 6973.788739]  blk_complete_reqs+0x35/0x50
[ 6973.789456]  __do_softirq+0xd7/0x2c8
[ 6973.790114]  ? sort_range+0x20/0x20
[ 6973.790763]  run_ksoftirqd+0x2a/0x40
[ 6973.791400]  smpboot_thread_fn+0xb5/0x150
[ 6973.792114]  kthread+0x10b/0x130
[ 6973.792724]  ? set_kthread_struct+0x50/0x50
[ 6973.793491]  ret_from_fork+0x1f/0x40

Fix this by increasing and decreasing active_io for each bio with
md_io_acct so that mddev_suspend() will wait until all bios from
io_acct_set finish before freeing io_acct_set.

Reported-by: Fine Fan <ffan@redhat.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>