Pull up following revision(s) (requested by ozaki-r in ticket #826):

sys/net/if_bridge.c: revision 1.155
sys/net/if.c: revision 1.421
sys/net/bpf.c: revision 1.224
sys/net/if.c: revision 1.422
sys/net/if.c: revision 1.423

Use if_is_mpsafe (NFC)

Protect packet input routines with KERNEL_LOCK and splsoftnet
if_input, i.e, ether_input and friends, now runs in softint without any
protections.  It's ok for ether_input itself because it's already MP-safe,
however, subsequent routines called from it such as carp_input and agr_input
aren't safe because they're not MP-safe.  Protect if_input with KERNEL_LOCK.
if_input can be called from a normal LWP context.  In that case we need to
prevent interrupts (softint) from running by splsoftnet to protect
non-MP-safe
codes (e.g., carp_input and agr_input).

Pointed out by mlelstv@

Protect if_deferred_start_softint with KERNEL_LOCK if the interface isn't
MP-safe

(martin)