lib/socket: autodetect RSS using ETHTOOL_GRXRINGS

lib/socket: autodetect RSS using ETHTOOL_GRXRINGS

This is also used as part of 'ethtool -n rdma14' and
'ethtool -x rdma14'.

;#> ethtool -n rdma14
8 RX rings available
rxclass: Cannot get RX class rule count: Operation not supported
RX classification rule retrieval failed

;#> ethtool -x rdma14
RX flow hash indirection table for rdma14 with 8 RX ring(s):
    0:      0     1     2     3     4     5     6     7
    8:      0     1     2     3     4     5     6     7
RSS hash key:
Operation not supported
RSS hash function:
    toeplitz: on
    xor: off
    crc32: off

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>