[SCSI] tgt: fix warning

Using scsi_tgt_lib in a new target module, we were getting
the following warning and a stack traceback on every I/O completion:

WARNING: at block/blk-core.c:1108

Which is claiming we may be leaking a bio.
We don't leak bios (blk_rq_unmap_user should free them).

Set rq->bio to NULL before calling scsi_host_put_command().
This was as advised by Fujita Tomonori.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>