lib/route/cls/u32.c: avoid overflowing an unsigned char

lib/route/cls/u32.c: avoid overflowing an unsigned char

If rtnl_u32_add_key is called too many times, sel->nkeys will wrap from
255 to 0, effectively killing all the previous rtnl_u32_add_key
calls (while having an inaccessible chunk of memory sitting beyond the
accessible part of the ->keys array). Not sure NOMEM is the best error
code, but that's at least something users would already have to be
prepared to handle.