features/shard: Fix EIO error on add-brick

        Backport of: https://review.gluster.org/14419

DHT seems to link inode during lookup even before initializing
inode ctx with layout information, which comes after
directory healing.

Consider two parallel writes. As part of the first write,
shard sends lookup on .shard which in its return path would
cause DHT to link .shard inode. Now at this point, when a
second write is wound, inode_find() of .shard succeeds and
as a result of this, shard goes to create the participant
shards by issuing MKNODs under .shard. Since the layout is
yet to be initialized, mknod fails in dht call path with EIO,
leading to VM pauses.

The fix involves shard maintaining a flag to denote whether
a fresh lookup on .shard completed one network trip. If it
didn't, all inode_find()s in fop path will be followed by a
lookup before proceeding with the next stage of the fop.

Big thanks to Raghavendra G and Pranith Kumar K for the RCA
and subsequent inputs and feedback on the patch.

Change-Id: Ibe59f6804a9c2ec95fbeaef1dc26858f16b8fcb5
BUG: 1426508
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: https://review.gluster.org/16748
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Smoke: Gluster Build System <jenkins@build.gluster.org>

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