cpumask: convert drivers/net/sfc

Impact: reduce stack usage, use new cpumask API.

Remove a cpumask from the stack.  Ben Hutchings indicated that printing
a warning and returning 1 was acceptable for the corner case where allocation
fails.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: linux-net-drivers@solarflare.com