f2fs: cleanup the f2fs_bio_alloc routine

Do cleanup more for better code readability.

- Change the parameter set of f2fs_bio_alloc()
  This function should allocate a bio only since it is not something like
  f2fs_bio_init(). Instead, the caller should initialize the allocated bio.

- Introduce SECTOR_FROM_BLOCK
  This macro translates a block address to its sector address.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index 2c445f8..0948405 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -82,6 +82,9 @@
 	(BITS_TO_LONGS(nr) * sizeof(unsigned long))
 #define TOTAL_SEGS(sbi)	(SM_I(sbi)->main_segments)
 
+#define SECTOR_FROM_BLOCK(sbi, blk_addr)				\
+	(blk_addr << ((sbi)->log_blocksize - F2FS_LOG_SECTOR_SIZE))
+
 /* during checkpoint, bio_private is used to synchronize the last bio */
 struct bio_private {
 	struct f2fs_sb_info *sbi;