drivers/IB,usnic: reduce scope of mmap_sem

usnic_uiom_get_pages() uses gup_longterm() so we cannot really get rid of
mmap_sem altogether in the driver, but we can get rid of some complexity
that mmap_sem brings with only pinned_vm.  We can get rid of the wq
altogether as we no longer need to defer work to unpin pages as the
counter is now atomic. We also share the lock.

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