LU-4684 ptlrpc: add dir migration connect flag

LU-4684 ptlrpc: add dir migration connect flag

Add dir migration connect flag to prevent collision with other
features. Though dir migration code exists, it will be reworked,
and the new RPC protocol won't be compatible with current one.

Also handle the previously-added OBD_CONNECT2_FLR flag.

Test-Parameters: trivial
Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Ie3eaec8602664170b52e3778b76059a0899ee7b1
Reviewed-on: https://review.whamcloud.com/31914
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>