null_blk: Add support for init_hctx() fault injection

This makes it possible to test the error path in blk_mq_realloc_hw_ctxs()
and also several error paths in null_blk.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Johannes Thumshirn <jth@kernel.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>