IB/iser: remove redundant macro definitions

Use the general linux definition for 4K and retrieve the rest from it.

Link: https://lore.kernel.org/r/1569359148-12312-1-git-send-email-maxg@mellanox.com


Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>