Description of problem: glfs_init called  with volume name prefixed by '/'  returns -1 and sets errno to 0.


Version-Release number of selected component (if applicable): 3.12


How reproducible: always


Steps to Reproduce:
1. Create some gluster volume, for example 'test'
2. Try to call glfs_init with volume name '/test'

Actual results: glfs_init returns -1 and sets errno to 0. With logging enabled it will log the following error: 
  E [MSGID: 101091] [graph.c:493:glusterfs_graph_prepare] 0-graph: glusterfs graph settop failed

Expected results: glfs_init should set errno to some meaningful value and correctly report volume name incorrectness.