quota: decouple fs reserved space from quota reservation

Currently inode_reservation is managed by fs itself and this
reservation is transfered on dquot_transfer(). This means what
inode_reservation must always be in sync with
dquot->dq_dqb.dqb_rsvspace. Otherwise dquot_transfer() will result
in incorrect quota(WARN_ON in dquot_claim_reserved_space() will be
triggered)
This is not easy because of complex locking order issues
for example http://bugzilla.kernel.org/show_bug.cgi?id=14739

The patch introduce quota reservation field for each fs-inode
(fs specific inode is used in order to prevent bloating generic
vfs inode). This reservation is managed by quota code internally
similar to i_blocks/i_bytes and may not be always in sync with
internal fs reservation.

Also perform some code rearrangement:
- Unify dquot_reserve_space() and dquot_reserve_space()
- Unify dquot_release_reserved_space() and dquot_free_space()
- Also this patch add missing warning update to release_rsv()
  dquot_release_reserved_space() must call flush_warnings() as
  dquot_free_space() does.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>