nilfs2: optimize rec_len functions

This is a similar change to those in ext2/ext3 codebase (commit
40a063f669 and a4ae309486, respectively).

The addition of 64k block capability in the rec_len_from_disk and
rec_len_to_disk functions added a bit of math overhead which slows
down file create workloads needlessly when the architecture cannot
even support 64k blocks.  This will cut the corner.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/ioctl.c

@@ -28,6 +28,7 @@
 #include <linux/vmalloc.h>
 #include <linux/compat.h>	/* compat_ptr() */
 #include <linux/mount.h>	/* mnt_want_write(), mnt_drop_write() */
+#include <linux/buffer_head.h>
 #include <linux/nilfs2_fs.h>
 #include "nilfs.h"
 #include "segment.h"