gfapi: handle root (/) resolution gracefully for realpath()

Resolving path of / is crucial for realpath() to work properly.

Change-Id: I8a1b482beba37f01afbd20446d9e6f6125f2ec34
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5152
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>