Fix PINNED routes handling.
Before r291643, adding new interface prefix had the following logic:
try_add:

EEXIST && (PINNED) {
  try_del(w/o PINNED flag)
  if (OK)
    try_add(PINNED)

}

In r291643, deletion was performed w/ PINNED flag held which leaded

to new interface prefixes (like ::1) overriding older ones.
Fix this by requesting deletion w/o RTF_PINNED.

PR: kern/205285
Submitted by: Fabian Keil <fk at fabiankeil.de>