This fixes a long standing problem where addresses added with non /128
prefixes and non infinite address lifetimes would register a prefix route
which would expire. Subsequent calls set new lifetimes for the same address
would not affect the prefix route management, so once expired, the
prefix route would be impossible to add back as the kernel would remove it.
Based on NetBSD changes (and reusing the commit message):
Also fixes a panic with INVARIANTS set and running "ndp -P"
after an IPv6 address is manually added and the same prefix
for the address is learned via router advertisment.
#11 0xffffffff805d42b0 in kassert_panic (fmt=0xffffffff80939e3a "prefix %p has referencing addresses")
#12 0xffffffff807d4049 in nd6_prefix_del (pr=0xfffff800043c0400) at /usr/src/sys/netinet6/nd6_rtr.c:1189
#13 0xffffffff807caf38 in nd6_ioctl (cmd=<optimized out>, data=<optimized out>, ifp=0xfffff80002337000)