RDMA/uverbs: Hoist the common process of disassociate_ucontext into ib core

This patch hoisted the common process of disassociate_ucontext
callback function into ib core code, and these code are common
to ervery ib_device driver.

Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>