Add MAC stats lock in xstats_get_by_id() callback before reading
number of supported MAC stats.
Fixes: 73280c1e4ff ("net/sfc: support xstats retrieval by ID")
Cc: stable@dpdk.org
Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>