Revert "ext4: fix fsx truncate failure"

This reverts commit 189e868f.

This commit reintroduces the use of ext4_block_truncate_page() in ext4
truncate operation instead of ext4_discard_partial_page_buffers().

The statement in the commit description that the truncate operation only
zero block unaligned portion of the last page is not exactly right,
since truncate_pagecache_range() also zeroes and invalidate the unaligned
portion of the page. Then there is no need to zero and unmap it once more
and ext4_block_truncate_page() was doing the right job, although we
still need to update the buffer head containing the last block, which is
exactly what ext4_block_truncate_page() is doing.

Moreover the problem described in the commit is fixed more properly with
commit

15291164
	jbd2: clear BH_Delay & BH_Unwritten in journal_unmap_buffer

This was tested on ppc64 machine with block size of 1024 bytes without
any problems.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>