staging/rdma/hfi1: Improve performance of user SDMA

To facilitate locked page counting, the user SDMA
routines would maintain a list of io vectors, which
were freed in the completion callback and then unpin
the associated pages during the next call into the
kernel.

Since the size of this list was unbounded, doing this
was bad for performance because the driver ended up
spending too much time freeing the io vectors.

This commit changes how the io vector freeing is done
by moving the actual page unpinning in the callback and
maintaining a count of unpinned pages. This count can
then be used during the next call into the kernel to
update the mm->pinned_vm variable (since that requires
process context and the ability to sleep.)

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>