dm-verity: use softirq context only when !need_resched()

Further limit verification in softirq (a.k.a. BH) context to cases where
rescheduling of the interrupted task is not pending.

This helps prevent the CPU from spending too long in softirq context.

Note that handle_softirqs() in kernel/softirq.c already stops running
softirqs in this same case.  However, that check is too coarse-grained,
since many I/O requests can be processed in a single BLOCK_SOFTIRQ.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>