md: fix an incorrect NULL check in md_reload_sb

md: fix an incorrect NULL check in md_reload_sb

The bug is here:
	if (!rdev || rdev->desc_nr != nr) {

The list iterator value 'rdev' will *always* be set and non-NULL
by rdev_for_each_rcu(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element
found (In fact, it will be a bogus pointer to an invalid struct
object containing the HEAD). Otherwise it will bypass the check
and lead to invalid memory access passing the check.

To fix the bug, use a new variable 'iter' as the list iterator,
while using the original variable 'pdev' as a dedicated pointer to
point to the found element.

Cc: stable@vger.kernel.org
Fixes: 70bcecdb1534 ("md-cluster: Improve md_reload_sb to be less error prone")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>