Add new APIs to iterate through the vdev peer list
by taking the peer reference.
A call back has to be passed to these APIs which will
be called for every peer in the lisst
dp_vdev_iterate_peer() -> iterates through peer_list of vdev
dp_pdev_iterate_peer() -> iterates through peer_list of all
vdevs in pdev
dp_soc_iterate_peer() -> iterates through peer_lists of all vdevs
in soc
Additonal APIs are added to for itearations where the callback
will be called outside the vdev->peer_list_lock, as these APIs
have additional memory allocations suggested to use only in case
the iteration need to happen outside lock
dp_vdev_iterate_peer_lock_safe()
dp_pdev_iterate_peer_lock_safe()
dp_soc_iterate_peer_lock_safe
Change-Id: I24632fe611355cc3e93b7f16d90913d4b8686ca9
e2b00339f3