quota: RFC add extend quota type v2

Currently only USER/GROUP quota are supported bu linux quota
But sometimes it is reasonable to have other types of quota
For example:
1) project_id in XFS
2) container_id in container(chroot) solutions.
This patch series is aimed to extend quota interface to support
different quota types. Last patch add the SUBTREE quota type.
This patch series does not contain fs-related part of
subtree-quota-type support. This is because generic subtree
support is relatively independent from quota part.
And in fact patches depends on two different devel trees
linux-fs.2.6 and ext4.git. So i've divided big subtree patchset
in to generic fs-related and quota-related parts.

First tree patches performs necessary cleanup work, And
the last one extends quota to support new quota's type.

Changes from v1:
 - fixes suggested by Jan Kara
 - redesign get_id() callback. Initially i've overlooked a quota transfer
   case. We have to pass new id's array to get_id() and it will return
   corresponding quota id. 
 - fix hard-coded values in dquot_initialize()

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
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