IB/uverbs: Refactor kern_spec_to_ib_spec_filter

The current implementation of kern_spec_to_ib_spec_filter, which takes
a uAPI based flow steering specification and creates the respective kernel
API flow steering structure, gets a ib_uverbs_flow_spec structure.
The new flow_action uAPI gets a match mask and filter from user-space
which aren't encoded in the flow steering's ib_uverbs_flow_spec structure.
Exporting the logic out of kern_spec_to_ib_spec_filter to get user-space
blobs rather than ib_uverbs_flow_spec structure.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>