block: Remove extra part pointer NULLify in blk_rq_init()

The rq->part pointer is already NULLified in the memset() call, so - like
for other pointers in rq - don't re-NULLify.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241125100258.4172774-1-john.g.garry@oracle.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>