RDMA: Remove rdma_user_mmap_page

Upon further research drivers that want this should simply call the core
function vm_insert_page(). The VMA holds a reference on the page and it
will be automatically freed when the last reference drops. No need for
disassociate to sequence the cleanup.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>