sfc: Fix assertions in efx_filter_rfs()

This function is intended to assert (when DEBUG is defined) that the
skb header area includes the header fields it's looking at, which RFS
should already have pulled.  But it uses pskb_may_pull(), which will
attempt to pull more data if necesary.  It must instead compare
skb_headlen() with the required length.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>