IB/qib: Fix local access validation for user MRs

Commit 8aac4cc3a9d7 ("IB/qib: RCU locking for MR validation") introduced
a bug that broke user post sends.  The proper validation of the MR
was lost in the patch.

This patch corrects that validation.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>