LU-10192 osd-zfs: create agent entry for remote entry

LU-10192 osd-zfs: create agent entry for remote entry

In DNE environment, the object (in spite of regular file
or directory) and its name entry may reside on different
MDTs. Under such case, we will create an agent entry on
the MDT where the object resides. The agent entry references
the object locally, that makes the object to be visible to
the userspace when mounted as 'zfs' directly. Then the
userspace tools, such as 'tar' can handle the object properly.
That is compatibile between ldiskfs backend and ZFS backend.

We handle the agent entry during set linkEA that is the common
interface for both regular file and directroy, can handle kinds
of cases, such as create/link/unlink/rename, and so on.

NOTE: we can NOT do that when ea_{insert,delete} that is only
for directory.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Icc4a63027221edf279994fbecda4d47cc121b799
Reviewed-on: https://review.whamcloud.com/29617
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>