blk-mq: add helper for checking if one CPU is mapped to specified hctx

Commit a46c2702 ("blk-mq: don't schedule block kworker on isolated CPUs")
rules out isolated CPUs from hctx->cpumask, and hctx->cpumask should only be
used for scheduling kworker.

Add helper blk_mq_cpu_mapped_to_hctx() and apply it into cpuhp handlers.

This patch avoids to forget clearing INACTIVE of hctx state in case that one
isolated CPU becomes online, and fixes hang issue when allocating request
from this hctx's tags.

Cc: Raju Cheerla <rcheerla@redhat.com>
Fixes: a46c2702

 ("blk-mq: don't schedule block kworker on isolated CPUs")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240517020514.149771-1-ming.lei@redhat.com
Tested-by: Raju Cheerla <rcheerla@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>