RDMA/ib_srp: Fix a deadlock

Remove the flush_workqueue(system_long_wq) call since flushing
system_long_wq is deadlock-prone and since that call is redundant with a
preceding cancel_work_sync()

Link: https://lore.kernel.org/r/20220215210511.28303-3-bvanassche@acm.org
Fixes: ef6c49d8

 ("IB/srp: Eliminate state SRP_TARGET_DEAD")
Reported-by:  <syzbot+831661966588c802aae9@syzkaller.appspotmail.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>