ext4: add project quota support

This patch adds mount options for enabling/disabling project quota
accounting and enforcement. A new specific inode is also used for
project quota accounting.

[ Includes fix from Dan Carpenter to crrect error checking from dqget(). ]

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>