IB/iser: Place the fmr pool into a union in iser's IB conn struct

This is preparation step for other memory registration methods to be
added.  In addition, change reg/unreg routines signature to indicate
they use FMRs.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>