RDMA/nes: Improve use of PBLs

Two level 256 byte PBLs was not implemented so the driver could report
out of memory when in fact there were PBLs still available.

This solution prefers to use 4KB PBLs over two level 256B PBLs until
the number of 4KB PBLs falls below a threshold.  At this point the 4KB
PBL structure is converted to use 256B PBLs which prevents the driver
from running out of 4KB PBLs too quickly.

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