features/snapview-server: check if the reference to the snapshot world is correct before doing any fop

The following operations might lead to problems:
* Create a file on the glusterfs mount point
* Create a snapshot (say "snap1")
* Access the contents of the snapshot
* Delete the file from the mount point
* Delete the snapshot "snap1"
* Create a new snapshot "snap1"

Now accessing the new snapshot "snap1" gives problems. Because the inode and
dentry created for snap1 would not be deleted upon the deletion of the snapshot
(as deletion of snapshot is a gluster cli operation, not a fop). So next time
upon creation of a new snap with same name, the previous inode and dentry itself
will be used. But the inode context contains old information about the glfs_t
instance and the handle in the gfapi world. Directly accessing them without
proper check leads to ENOTCONN errors. Thus the glfs_t instance should be
checked before accessing. If its wrong, then right instance should be obtained
by doing the lookup.

Change-Id: I975245b8f6b7fea0a90eb5e36e8149d12457ac10
BUG: 1158791
Reviewed-on: http://review.gluster.org/9007
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>