RDMA/i40w: Hold read semaphore while looking after VMA

VMA lookup is supposed to be performed while mmap_sem is held.

Fixes: f26c7c83 ("i40iw: Add 2MB page support")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>