block: fix oops on !disk->queue and sysfs discard alignment display

block: fix oops on !disk->queue and sysfs discard alignment display

Eric Dumazet reports:

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At boot, I have a crash in part_discard_alignment_show+0x1b/0x50

CR2 : 000006ac

fault in : mov    0x2c(%rcx),%edx

I suspect commit b79f3800769c962bf3 (block: Remove extra
discard_alignment from hd_struct) being in fault

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Not quite known how ->queue can be NULL while the sysfs entry
exists, but lets play it safe and check for a NULL queue.
The rest of the sysfs show strategies in check.c do not dereference
disk->queue.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>