f2fs: clean up with a macro

This patch adds GET_BLKOFF_FROM_SEG0 to clean up some codes.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index 5731682..4024546 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -57,6 +57,9 @@
 	((blk_addr) - SM_I(sbi)->seg0_blkaddr)
 #define GET_SEGNO_FROM_SEG0(sbi, blk_addr)				\
 	(GET_SEGOFF_FROM_SEG0(sbi, blk_addr) >> sbi->log_blocks_per_seg)
+#define GET_BLKOFF_FROM_SEG0(sbi, blk_addr)				\
+	(GET_SEGOFF_FROM_SEG0(sbi, blk_addr) & (sbi->blocks_per_seg - 1))
+
 #define GET_SEGNO(sbi, blk_addr)					\
 	(((blk_addr == NULL_ADDR) || (blk_addr == NEW_ADDR)) ?		\
 	NULL_SEGNO : GET_L2R_SEGNO(FREE_I(sbi),			\