make snapview-server more compatible with NFS server

* There was no handle based API for listxattr. With this change, glfs_h_getxattrs
  also handles the listxattr functionality by checking whether the name is NULL
  or not (like posix). But all the gfapi functions for listxattr
  (glfs_h_getxattrs AND glfs_listxattr AND glfs_flistxattr) returns the names of
  the xattrs in a buffer provided by the caller. But snapview-server has to
  return the list of xattrs in a dict itself (similar to posix xlator). But
  the buffer just contains the names of the xattrs. So for each xattr, a zero
  byte value is set (i.e. "") into the dict and sent back. Translators which
  do xattr caching (as of now md-cache which caches selinux and acl related
  xattrs) should not cache those xattrs whose value is a zero byte data ("").
  So made changes in md-cache to ignore zero byte values.

* NFS server was not linking the inodes to inode table in readdirp. This was
  leading to applications getting errors. The below set of operations would
  lead to applications getting error
  1) ls -l in one of the snaopshots (snapview-server would generate gfids for
     each entry on the fly and link the inodes associated with those entries)
  2) NFS server upon getting readdirp reply would not link the inodes of the
     entries. But it used to generate filehandles for each entry and associate
     the gfid of that entry with the filehandle and send it as part of the
     reply to nfs client.
  3) NFS client would send the filehandle of one of those entries when some
     activity is done on it.
  4) NFS server would not be able to find the inode for the gfid present in the
     filehandle (as the inode was not linked) and would go for hard resolution
     by sending a lookup on the gfid by creating a new inode.
  5) snapview-client will not able to identify whether the inode is a real inode
     existing in the main volume or a virtual inode existing in the snapshots
     as there would not be any inode context.
  6) Since the gfid upon which lookup is sent is a virtual gfid which is not
     present in the disk, lookup would fail and the application would get an
     error.

  To handle above situation, now nfs server also does inode linking in readdirp.

Change-Id: Ibb191408347b6b5f21cff72319ccee619ea77bcd
BUG: 1115949
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/8230
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>