RDMA/nes: Account for freed PBL after HW operation

Fix occurrences where the software PBL counts were changed before the
hardware was updated.  This bug allowed another thread to overallocate
the hardware resources.

Add proper PBL accounting in case nes_reg_mr() fails.

Signed-off-by: Don Wood <donald.e.wood@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>