LU-10810 ptlrpc: introduce OST_SEEK RPC

LU-10810 ptlrpc: introduce OST_SEEK RPC

For the purposes of SEEK_HOLE/SEEK_DATA support introduce
new OST_SEEK RPC.

Patch add RPC layout, unified handler and connect flag for
compatibility needs.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I1580902b6b773d9a6d6f9beaa1ee1da60fbc20f8
Reviewed-on: https://review.whamcloud.com/39707
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>