Add support for backchannels to the kernel RPC. Backchannels
are used by NFSv4.1 for callbacks. A backchannel is a connection
established by the client, but used for RPCs done by the server
on the client (callbacks). As a result, this patch mixes some
client side calls in the server side and vice versa. Some
definitions in the .c files were extracted out into a file called
krpc.h, so that they could be included in multiple .c files.
This code has been in projects/nfsv4.1-client for some time.
Although no one has given it a formal review, I believe kib@
has taken a look at it.
Rework flow control for connection-oriented (TCP) RPC server.

  When processing receive buffer, write the amount of data, expected
in present request record, into socket's so_rcv.sb_lowat to make stack
aware about our needs.  When processing following upcalls, ignore them
until socket collect enough data to be read and processed in one turn.
  This change reduces number of context switches and other operations
in RPC stack during large NFS writes (especially via non-Jumbo networks)
by order of magnitude.

  After precessing current packet, take another look into the pending
buffer to find out whether the next packet had been already received.
If not, deactivate this port right there without making RPC code to
push this port to another thread just to find that there is nothing.
If the next packet is received partially, also deactivate the port, but
also update socket's so_rcv.sb_lowat to not be woken up prematurely.
  This change additionally reduces number of context switches per NFS
request about in half.

(cherry picked from commit 17e956838c06da00cf97085488d43b4f72b9b34d)

Conflicts:
	sys/rpc/svc_vc.c
Some minor tuning to rpc/svc.c:
 - close cosmetic race in svc_exit();
 - do not set wait timeout for idle threads if we have no use for wakeups;
 - create new requested thread sooner, not only after some another thread
wakeup, that may happen later under constant load.
Remove several linear list traversals per request from RPC server code.

  Do not insert active ports into pool->sp_active list if they are success-
fully assigned to some thread.  This makes that list include only ports that
really require attention, and so traversal can be reduced to simple taking
the first one.

  Remove idle thread from pool->sp_idlethreads list when assigning some
work (port of requests) to it.  That again makes possible to replace list
traversals with simple taking the first element.

(cherry picked from commit 88262ed57f27358d620b3a57a2ee02d35ca34825)
Fix build.

(cherry picked from commit 303df467d63af2982ee17262210aae6673ab711e)
Fix RPC server threads file handle affinity to work better with ZFS.

  Instead of taking 8 specific bytes of file handle to identify file during
RPC thread affitinity handling, use trivial hash of the full file handle.
ZFS's struct zfid_short does not have padding field after the length field,
as result, originally picked 8 bytes are loosing lower 16 bits of object ID,
causing many false matches and unneeded requests affinity to same thread.
  This fix substantially improves NFS server latency and scalability in SPEC
NFS benchmark by more flexible use of multiple NFS threads.

Sponsored by:	iXsystems, Inc.

(cherry picked from commit c84160c1b25c50cb41c73db768aba34d5cb7c448)