IB/ipath: Improve UD loopback performance by allocating temp array only once

Receive work queue entries are checked for L_Key validity, and
pointers to the memory region structure are saved in an allocated
structure.  For UD loopback packets, this structure is allocated and
freed for each packet.  This patch changes that to allocate/free
during QP creation and destruction.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>