IB/iser: Push the decision what memory key to use into fast_reg_mr routine

This is a preparation step for T10-PI offload support.  We prefer to
push the desicion of which mkey to use (global or fastreg) to
iser_fast_reg_mr.  We choose to do this since in T10-PI we may need to
register for protection buffers and in this case we wish to simplify
iser_fast_reg_mr instead of repeating the logic of which key to use.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Alex Tabachnik <alext@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>