IB/core: Rename 'flow' structs to match other uverbs structs

Commit 436f2ad05a0b ("IB/core: Export ib_create/destroy_flow through
uverbs") added public data structures to support receive flow
steering.  The new structs are not following the 'uverbs' pattern:
they're lacking the common prefix 'ib_uverbs'.

This patch replaces ib_kern prefix by ib_uverbs.

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>