MFC r324752: Relax per-ifnet cif_vrs list double locking in carp(4).

In all cases where cif_vrs list is modified, two locks are held: per-ifnet
CIF_LOCK and global carp_sx. It means to read that list only one of them
is enough to be held, so we can skip CIF_LOCK when we already have carp_sx.

This fixes kernel panic, caused by attempts of copyout() to sleep while
holding non-sleepable CIF_LOCK mutex.