update documentation for the new truncate sequence

deliberate; as soon as struct block_device * is propagated in a reasonable

way by that code fixing will become trivial; until then nothing can be

done.

[mandatory]

	block truncatation on error exit from ->write_begin, and ->direct_IO

moved from generic methods (block_write_begin, cont_write_begin,

nobh_write_begin, blockdev_direct_IO*) to callers.  Take a look at

ext2_write_failed and callers for an example.

[mandatory]

	->truncate is going away.  The whole truncate sequence needs to be

implemented in ->setattr, which is now mandatory for filesystems

implementing on-disk size changes.  Start with a copy of the old inode_setattr

and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to

be in order of zeroing blocks using block_truncate_page or similar helpers,

size update and on finally on-disk truncation which should not fail.

inode_change_ok now includes the size checks for ATTR_SIZE and must be called

in the beginning of ->setattr unconditionally.