IB/iser: Rename iser_reg_page_vec to iser_fast_reg_fmr

Also, change a name of a local variable.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Adir Lev <adirl@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>