ifnet: Make ifnet and ifindex2ifnet MPSAFE

ifnet: Make ifnet and ifindex2ifnet MPSAFE

- Accessing to these two global variables from non-netisr threads uses
  ifnet lock.  This kind of accessing is from
- Accessing to ifindex2ifnet from netisrs are lockless MPSAFE.
- Netisrs no longer access ifnet, instead they access ifnet array as of
  this commit, which is lockless MPSAFE.

Rules for accessing ifnet and ifindex2ifnet is commented near the
declaration of the related global variables/functions in net/if_var.h.