md: recheck spare changes before starting sync

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

md: recheck spare changes before starting sync

remove_spares() and remove_and_add_spares() modify the array's rdev
configuration. These operations are only safe after the array has been
suspended.

md_start_sync() checks whether spare configuration changes are needed
before taking reconfig_mutex. However, the rdev state can change before
the mutex is acquired, so the initial check can become stale. In that
case, md_choose_sync_action() may remove or replace rdevs while normal
I/O is still accessing them.

The race can occur as follows:

raid10d Worker Normal IO
____________ _______________________ ______________________

raid10_write_request()
wait_blocked_dev()
set Blocked
set Faulty
Skip Faulty rdev
rrdev->nr_pending++
.repl_bio = bio
removeable_rdev = false .
array not suspended .
lock mddev goto err_handle
lock mddev (wait)
.
update sb .
clear Blocked .
.
unlock mddev .
lock mddev (acquires)
remove_spares()
removeable_rdev = true

raid10_remove_disk()
rdev = replacement
replacement = NULL
rdev_dec_pending(NULL)
unlock mddev (NULL)->nr_pending--

In this case, rdev_dec_pending() is called with a NULL pointer,
resulting in a NULL pointer dereference when attempting to decrement
nr_pending.

Fix this by suspending the array when spare configuration changes are
needed, including for non-read-write arrays, and checking again after
taking reconfig_mutex. If the array was not already suspended and a
change is now needed, release the mutex, suspend the array, and
reacquire the mutex before continuing.