net: fix net_reopen() initialization

net: fix net_reopen() initialization

Mantas Mikulenas reviewed recent pull reques that contained commit
b90a522f23167bd00062504803e94220937aba23 making a code path quicker.
Unfortunately initialization was incomplete, as demonstrated by R.E.
Wolff in same review.  Fix is to use GCC designated initializer.

Reference: https://github.com/traviscross/mtr/pull/151
Reference: https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html

Review update.  Replace following initializer with an iterative loop.

  .saved = { [0 ... SAVED_PINGS - 1] = -2 },

This is done to keep compatibility with non-gcc compilers.

Reference: https://github.com/traviscross/mtr/pull/153