fuse: allow xlators to request for direct-io-mode on virtual files

Translators like meta, create virtual files with dynamic content
generated only at the time of open(). Therefore the file size
returned in lookup or stat is 0 (just like files in /proc). However
the VFS does not read beyond the size, and if the size is 0, no
READ ever reaches gluster for that file -- unless direct-io-mode
is enabled.

This patch allows translators to return "direct-io-mode" flag for
such 0-byte virtual files in xdata of open_cbk/create_cbk.

Change-Id: I3fe3312cd96baa4eecfe1247ab7255b4f455f049
BUG: 1089216
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/7506
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>