vfs_glusterfs: Add comments to the pipe(2) code.

vfs_glusterfs: Add comments to the pipe(2) code.

The guarantees around read(2) and write(2) and pipes are critical
to understanding this code.  Hopefully these comments will help.

Signed-off-by: Ira Cooper <ira@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jan 23 20:58:51 CET 2015 on sn-devel-104

(cherry picked from commit 3b2d8bdbb1851961536241d3aaaf6ac936964517)

The last 10 patches address
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11069
Backport remaining performance patches for vfs_glusterfs to 4.2/4.1.