LU-11130 osd-ldiskfs: create non-empty local agent symlinks

LU-11130 osd-ldiskfs: create non-empty local agent symlinks

e2fsck doesn't like zero-sized symlink inodes created by
osd_create_local_agent_inode().  Store the FID of the remote
inode as the symlink target so that it is possible to debug
where this symlink comes from in case there is some problem
in the future.

It would be better to just migrate the whole symlink instead
of creating a remote symlink, in the very common case where
there is not a hard link to the symlink (which POSIX allows,
but is extremely uncommon).  For the short term, we store
keep the remote symlinks to ensure on-disk consistency with
this simple patch, until the migration code can be fixed.

Lustre-change: https://review.whamcloud.com/32797
Lustre-commit: c3a836364892cacbc4737645893b094971c6ec49

Cray-bug-id: LUS-6189
Change-Id: Ida43616c51b6903f0a51aeec05a9a2dd189efe31
Signed-off-by: Alexander Zarochentsev <c17826@cray.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33973
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>