ethdev: clarify null location case in xstats get

ethdev: clarify null location case in xstats get

When xstats location is null in rte_eth_xstats_get() the return value
is not clearly specified.  Some PMDs (eg. hns3/ipn3ke/mvpp2/axgbe) return
zero while others return the required number of elements.

In this patch, special parameter combinations are restricted:
 1. highlight that xstats location may be null if and only if n is 0.
 2. amend n parameter description to specify that if n is lower than
    the required number of elements, the function returns the required
    number of elements.
 3. specify that if n is zero, the xstats must be NULL, the function
    returns the required number of elements (a duplicate which should
    help to not very attentive readers).

Add sanity check for null xstats and non-zero n case on API level to
make it unnecessary to care about it in drivers.

Fixes: ce757f5c9a4d ("ethdev: new method to retrieve extended statistics")
Cc: stable@dpdk.org
Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>