ext4: avoid unnecessarily writing back dirty pages before hole punching

ext4: avoid unnecessarily writing back dirty pages before hole punching

For hole punching, currently ext4 will synchronously write back the
dirty pages fit into the hole, since the data on the disk responding
to those pages are to be deleted, it is benefical to directly release
those pages, no matter they are dirty or not, except the ordered case.

[ Fixed error return to unlock i_mutex if ext4_begin_ordered_punch_hole()
  fails.  Thanks to Wei Yongjun <yongjun_wei@trendmicro.com.cn> for
  pointing this out.]

Signed-off-by: Li Wang <liwang@ubuntukylin.com>
Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Reviewed-by: Jan Kara <jack@suse.cz>