nilfs2: optimize rec_len functions

This is a similar change to those in ext2/ext3 codebase (commit
40a063f6 and a4ae3094, 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"

include/linux/nilfs2_fs.h

@@ -326,17 +326,21 @@ static inline unsigned nilfs_rec_len_from_disk(__le16 dlen)
 {
 	unsigned len = le16_to_cpu(dlen);
 
+#if !defined(__KERNEL__) || (PAGE_CACHE_SIZE >= 65536)
 	if (len == NILFS_MAX_REC_LEN)
 		return 1 << 16;
+#endif
 	return len;
 }
 
 static inline __le16 nilfs_rec_len_to_disk(unsigned len)
 {
+#if !defined(__KERNEL__) || (PAGE_CACHE_SIZE >= 65536)
 	if (len == (1 << 16))
 		return cpu_to_le16(NILFS_MAX_REC_LEN);
 	else if (len > (1 << 16))
 		BUG();
+#endif
 	return cpu_to_le16(len);
 }