iser-target: generalize rdma memory registration and cleanup

Current driver uses global dma key to register the memory pointed
by sg list provided by the target core.

This is the preparation step for adding more methods like fast
path memory registration, make the reg/unreg calls be function
pointers.

Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>