net/sfc/base: improve API to get supported filter matches

net/sfc/base: improve API to get supported filter matches

The previous API had various problems, including the length of the
caller provided buffer not being specified, no means being available
to discover how big the buffer needs to be, and a lack of clarity of
what the resulting list contains.

To improve it:
- add the buffer length as a parameter
- if the provided buffer is too short, fail with ENOSPC and return
  the required length
- ensure that the list contents are valid and add comments describing it

It is safe to change this API as, unsurprisingly, it has no users.

Signed-off-by: Mark Spender <mspender@solarflare.com>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>