IB/qib: Fix refcount leak in lkey/rkey validation

The mr optimization introduced a reference count leak on an exception
test.  The lock/refcount manipulation is moved down and the problematic
exception test now calls bail to insure that the lock is released.

Additional fixes as suggested by Ralph Campbell <ralph.campbell@qlogic.org>:
- reduce lock scope of dma regions
- use explicit values on returns vs. automatic ret value

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>