IB/qib: Change get_user_pages() usage to always NULL vmas

The static helper routine, __qib_get_user_pages(), accepts a vma arg,
but current use always passes NULL.

This has caused some confusion associated with the correct use of this
argument, but since the current use case doesn't require the
flexiblity, the best thing to do is to simplfy the code to always pass
NULL to get_user_pages().

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>