than setattr > vmtruncate > truncate, have filesystems call their truncate
sequence from ->setattr if filesystem specific operations are required. 
vmtruncate is deprecated, and truncate_pagecache and inode_newsize_ok
helpers introduced previously should be used.

simple_setattr is introduced for simple in-ram filesystems to implement
the new truncate sequence.  Eventually all filesystems should be converted
to implement a setattr, and the default code in notify_change should go
away.

simple_setsize is also introduced to perform just the ATTR_SIZE portion of
simple_setattr (ie.  changing i_size and trimming pagecache).

A new attribute is introduced into inode_operations structure;
.new_truncate is a temporary hack to distinguish filesystems that
implement the new truncate system.

To implement the new truncate sequence:
- set .new_truncate = 1
- filesystem specific manipulations (eg freeing blocks) must be done in
  the setattr method rather than ->truncate.
- vmtruncate can not be used by core code to trim blocks past i_size in
  the event of write failure after allocation, so this must be performed
  in the fs code.
- make use of the better opportunity to catch errors with the above 2 changes.
- inode_setattr should not be used. generic_setattr is a new function
  to be used to copy simple attributes into the generic inode.

Big problem with the previous calling sequence: the filesystem is not
called until i_size has already changed.  This means it is not allowed to
fail the call, and also it does not know what the previous i_size was. 
Also, generic code calling vmtruncate to truncate allocated blocks in case
of error had no good way to return a meaningful error (or, for example,
atomically handle block deallocation).
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Steven French <sfrench@us.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>