drivers/IB,core: reduce scope of mmap_sem

drivers/IB,core: reduce scope of mmap_sem

ib_umem_get() uses gup_longterm() and relies on the lock to stabilze the
vma_list, so we cannot really get rid of mmap_sem altogether, but now that
the counter is atomic, we can get of some complexity that mmap_sem brings
with only pinned_vm.

Link: http://lkml.kernel.org/r/20190206175920.31082-7-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>