IB/qib: Fix qib_mr structure

struct qib_mr requires the mr member be the last because struct
qib_mregion contains a dynamic array at the end.  The additions
of members should have been placed before this structure as the
comment noted.

Failure to do so was causing random memory corruption.  Reproducing
this bug was easy to do by running the client and server of
ib_write_bw -s 8 -n 5 on the same node.

This BUG() was tripped in a slab debug kernel:

kernel BUG at mm/slab.c:2572!

Fixes: 38071a46 ("IB/qib: Support the new memory registration API")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>