IB/core: Use a common header for uverbs flow_specs

A common header will allows better checking of flow specs size, while
ensuring strict alignment to 64 bits.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Link: http://marc.info/?i=cover.1383773832.git.ydroneaud@opteya.com
Signed-off-by: Roland Dreier <roland@purestorage.com>