RDMA/umem: Don't hold mmap_sem for too long

DMA mapping is time consuming operation and doesn't need to be performed
with mmap_sem semaphore is held.

The semaphore only needs to be held for accounting and get_user_pages
related activities.

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