IB/iser: Avoid FRWR notation, use fastreg instead

FRWR stands for "fast registration work request". We want to avoid
calling the fastreg pool with that name, instead we name it fastreg
which stands for "fast registration".

This pool will include more elements in the future, so it is a good
idea to generalize the name.

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