LU-5319 ptlrpc: Add a tag field to ptlrpc messages

LU-5319 ptlrpc: Add a tag field to ptlrpc messages

The new tag field is used as a virtual index for multiple modifying
RPCs management. It is set by the client and allows the target to
release in-memory reply data when the tag is reused by a new RPC.

The tag field replaces the unused last_seen field of ptlrpcd_body
structure.

Additionally, the last_xid field is used to transfer the highest XID
for which a reply has been received and does not have an unreplied
lower-numbered XID.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I4a57f3710ffffe21d8d655af6ac222b65051a12d
Reviewed-on: http://review.whamcloud.com/14095
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>