verify: fix potential overflow before widen

verify: fix potential overflow before widen

vc->hdr_num and len are both 32 bits wide and their product will be a
32-bit result. So any overflow will be lost. Cast hdr_num to unsigned
long long so that nothing is lost if the product overflows a 32-bit
integer.

This fixes the following issue reported by Coverity.

** CID 486274:  Integer handling issues  (OVERFLOW_BEFORE_WIDEN)
/verify.c: 347 in log_verify_failure()

________________________________________________________________________________________________________
*** CID 486274:  Integer handling issues  (OVERFLOW_BEFORE_WIDEN)
/verify.c: 347 in log_verify_failure()
341             uint32_t len;
342             struct thread_data *td = vc->td;
343
344             offset = vc->io_u->verify_offset;
345             if (td->o.verify != VERIFY_PATTERN_NO_HDR) {
346                     len = hdr->len;
>>>     CID 486274:  Integer handling issues  (OVERFLOW_BEFORE_WIDEN)
>>>     Potentially overflowing expression "vc->hdr_num * len" with type "unsigned int" (32 bits, unsigned) is evaluated using 32-bit arithmetic, and then used in a context that expects an expression of type "unsigned long long" (64 bits, unsigned).
347                     offset += vc->hdr_num * len;
348             } else {
349                     len = vc->io_u->buflen;
350             }
351
352             log_err("%.8s: verify failed at file %s offset %llu, length %u"

Fixes: 9c8b90ae ("fix wrong offset for VERIFY_PATTERN_NO_HDR")
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>