tpps: add check before tpps_del_queue()

We found panic on BUG_ON(tppg->nr_tppq < 1) in pressure-testing.
After long term (about one month) tracing we finally find out the reason: some scsi_cmds
passed from scsi_ioctl will call blk_execute_rq_nowait and call tpps_set_request() but
never call tpps_insert_request() (It even doesn't get into io-scheduler) so the counter
of nr_tppq gets wrong.
To fix it, we add check before calling tpps_del_queue() and for making it safer, adding
check before BUG_ON() to report more information.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Merge branch 'dev' of github.com:RobinDong/ali_kernel into dev