gfapi: provide stable st_dev in stat structure

Samba depends on st_dev being unique and stable for different
filesystems (it uses st_dev+st_ino as key to store metadata of
a file).

Change-Id: Ia022fabadfb5ef3fc9724c1b1ca86f1a20f98af9
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5164
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>