ip: Convert non-constant initializers to macros

ip: Convert non-constant initializers to macros

As per the C standard, "expressions in an initializer for an object that
has static or thread storage duration shall be constant expressions".
Aggregate objects are not constant expressions. Newer GCC doesn't mind, but
older GCC and LLVM do.

Therefore convert to a macro. And since all these macros will look very
similar, extract a generic helper, IPSTATS_STAT_DESC_XSTATS_LEAF, which
takes the leaf name as an argument and initializes the rest as appropriate
for an xstats descriptor.

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>