sfc: Hold efx_nic::stats_lock while reading efx_nic::mac_stats

efx_nic::stats_lock is used to serialise stats updates, but each
reader was dropping it before it finished reading efx_nic::mac_stats.

If there were concurrent stats reads using procfs, or one using procfs
and one using ethtool, an update could race with a read.  On a 32-bit
system, the reader could see word-tearing of 64-bit stats (32 bits of
the old value and 32 bits of the new).

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