RDMA/core: Provide rdma_user_mmap_disassociate() to disassociate mmap pages

RDMA/core: Provide rdma_user_mmap_disassociate() to disassociate mmap pages

Provide a new api rdma_user_mmap_disassociate() for drivers to
disassociate mmap pages for a device.

Since drivers can now disassociate mmaps by calling this api,
introduce a new disassociation_lock to specifically prevent
races between this disassociation process and new mmaps. And
thus the old hw_destroy_rwsem is not needed in this api.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://patch.msgid.link/20240927103323.1897094-2-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>