net: ipv6: Ignore reachable time greater than maximum

This commit fixes the crash of echo server from unsolicited RA with
reachable time set to 2147483648. The crash was in
net_if_ipv6_calc_reachable_time because such large value resulted in
modulus by zero due to integer overflow.

Fixes #6382

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>