ext4: make the zero-out chunk size tunable

Currently in ext4 the length of zero-out chunk is set to 7 file system
blocks.  But if an inode has uninitailized extents from using
fallocate to preallocate space, and the workload issues many random
writes, this can cause a fragmented extent tree that will
unnecessarily grow the extent tree.

So create a new sysfs tunable, extent_max_zeroout_kb, which controls
the maximum size where blocks will be zeroed out instead of creating a
new uninitialized extent.  The default of this has been sent to 32kb.

CC: Zach Brown <zab@zabbo.net>
CC: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

Documentation/ABI/testing/sysfs-fs-ext4

@@ -96,3 +96,16 @@ Contact:	"Theodore Ts'o" <tytso@mit.edu>
 Description:
 		The maximum number of megabytes the writeback code will
 		try to write out before move on to another inode.
+
+What:		/sys/fs/ext4/<disk>/extent_max_zeroout_kb
+Date:		August 2012
+Contact:	"Theodore Ts'o" <tytso@mit.edu>
+Description:
+		The maximum number of kilobytes which will be zeroed
+		out in preference to creating a new uninitialized
+		extent when manipulating an inode's extent tree.  Note
+		that using a larger value will increase the
+		variability of time necessary to complete a random
+		write operation (since a 4k random write might turn
+		into a much larger write due to the zeroout
+		operation).