scsi: ufs_test: fix use after free issue

Make sure test_rq is not accessed after its freed.
Also take care of initializing num_bios_per_request before use.

Change-Id: Ief9e4ff118691d74782f4f405dc195cb9bdb7c32
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
[cang@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>