IB/core: clarify overflow/underflow checks on ib_create/destroy_flow

This patch fixes the following issues:

1. Unneeded checks were removed

2. Removed the fixed size out of flow_attr.size, thus simplifying the checks.

3. Remove a 32bit hole on 64bit systems with strict alignment in
   struct ib_kern_flow_att by adding a reserved field.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>