LU-6349 ptlrpc: remove old protocol compatibility

Some old protocol compatibility workarounds are still present in
master that should have been removed when LUSTRE_MSG_MAGIC_V1 was.

In particular, the process for upgrading LUSTRE_MSG_MAGIC_V1 to
LUSTRE_MSG_MAGIC_V2 had the client to connect to the server with the
V1 protocol with op_flag=MSG_CONNECT_NEXT_VER set, and if the server
supported the V2 protocol it would reply with LUSTRE_MSG_MAGIC_V2.
This ensured that if the new client connected to an old server the
connection would be allowed.  However, even with V1 protocol support
removed, the 2.x clients are still connecting with NEXT_VER set.
In b1_8 this flag was contingent on LUSTRE_MSG_MAGIC_V1 being used,
which is how it should have been in 2.x as well.

A few other cleanups are be done at the same time:
- disallow 1.8 clients (or at least those that don't understand
  OBD_CONNECT_FULL20) so we can remove workarounds for 1.8 clients
- remove support for pre-2.1 DLM flock lock handling
- don't workaround the lack of MDS_ATTR_xTIME_SET flags in setattr
- always set MSGHDR_CKSUM_INCOMPAT18 (it can eventually be removed)

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Id0a7a5724f2f84decaa3444fe278d0fc05500c1e
Reviewed-on: http://review.whamcloud.com/14006
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>