gfapi: perform open() on pre-existing inode in glfs_creat()

Performing syncop_create() on a pre-existing file can result
in opening the linkfile if DHT has a linkfile for the file.
This is because dht_create() will perform the op on the
hashed_subvol() and overwrite the layout in dht_create_cbk.

Change-Id: I7a0db56921ec9fc3e278e0418db3b967f81e5598
BUG: 990410
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5884
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>