[SCSI] bnx2i: Fixed kernel panic due to illegal usage of sc->request->cpu

A kernel panic was observed when passing the sc->request->cpu = -1 to
retrieve the per_cpu variable pointer:
 #0 [ffff880011203960] machine_kexec at ffffffff81022bc3
 #1 [ffff8800112039b0] crash_kexec at ffffffff81088630
 #2 [ffff880011203a80] __die at ffffffff8139ea20
 #3 [ffff880011203aa0] no_context at ffffffff8102f3a7
 #4 [ffff880011203ae0] __bad_area_nosemaphore at ffffffff8102f665
 #5 [ffff880011203ba0] retint_signal at ffffffff8139dd1f
 #6 [ffff880011203cc8] bnx2i_indicate_kcqe at ffffffffa03dc4f2
 #7 [ffff880011203da8] service_kcqes at ffffffffa03cb04f
 #8 [ffff880011203e68] cnic_service_bnx2x_kcq at ffffffffa03cb14a
 #9 [ffff880011203e88] cnic_service_bnx2x_bh at ffffffffa03cb1b3

The problem lies in the slow path sg_io (and perhaps sg_scsi_ioctl) call to
blk_get_request->get_request/wait->blk_alloc_request->blk_rq_init which
re-initializes the request->cpu to -1.  There is no assignment for cpu from
that to the request_fn call to low level drivers.

When this happens, the sc->request->cpu will be using the init value of
-1.  This will create a kernel panic when it hits bnx2i because the code
refers it to get the per_cpu variables ptr.

This change is to put in a guard against that and also for cases when
bio affinity/queue completion to the same cpu is not enabled.  In those
cases, the request->cpu will remain a -1 also.

This bug was created from commit:  b5cf6b63

For the case when the blk layer did not setup the request->cpu, bnx2i
will complete the sc with the current CPU of the thread.

Signed-off-by: Eddie Wai <eddie.wai@broadcom.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>