md/raid5: don't index beyond end of array in need_this_block().
When need_this_block probably shouldn't be called when there are more than 2 failed devices, we really don't want it to try indexing beyond the end of the failed_num of fdev arrays. So limit the loops to at most 2 iterations. Reported-by: Shaohua Li <firstname.lastname@example.org> Signed-off-by: NeilBrown <email@example.com>
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