RDMA/cm: Pull duplicated code into cm_queue_work_unlock()

RDMA/cm: Pull duplicated code into cm_queue_work_unlock()

While unlocking a spinlock held by the caller is a disturbing pattern,
this extensively duplicated code is even worse. Pull all the duplicates
into a function and explain the purpose of the algorithm.

The on creation side call in cm_req_handler() which is different has been
micro-optimized on the basis that the work_count == -1 during creation,
remove that and just use the normal function.

Link: https://lore.kernel.org/r/20200506074701.9775-5-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>