sfc: Add use of shared RSS contexts.

Allow PFs to allocate shared RSS contexts if we exhaust our
exclusive RSS contexts. Make VFs use shared RSS contexts in
all cases.
Spruce up error handling so that the shadow copy of the RSS
table is updated after successful update, rather than in all
cases, so that we report the actual contents of the RSS table
after a failure to set it, rather than what we'd like it to be.

Populate context_size parameter when vacuously allocating RSS
context of size 1.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>