drivers/IB,qib: optimize mmap_sem usage

The driver uses mmap_sem for both pinned_vm accounting and
get_user_pages(). Because rdma drivers might want to use gup_longterm() in
the future we still need some sort of mmap_sem serialization (as opposed
to removing it entirely by using gup_fast()). Now that pinned_vm is atomic
the writer lock can therefore be converted to reader.

This also fixes a bug that __qib_get_user_pages was not taking into
account the current value of pinned_vm.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>