ext4: correctly handle pages w/o buffers in ext4_discard_partial_buffers()

If a page has been read into memory and never been written, it has no
buffers, but we should handle the page in truncate or punch hole.

VFS code of writing operations has handled holes correctly, so this
patch removes the code handling holes in writing operations.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org