f2fs: adjust the way of calculating nat block

use a slightly simpler expression to calculate nat block with nid.
Signed-off-by: NFan Li <fanofcode.li@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.h

@@ -200,13 +200,16 @@ static inline pgoff_t current_nat_addr(struct f2fs_sb_info *sbi, nid_t start)
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	pgoff_t block_off;
 	pgoff_t block_addr;
-	int seg_off;
 
+	/*
+	 * block_off = segment_off * 512 + off_in_segment
+	 * OLD = (segment_off * 512) * 2 + off_in_segment
+	 * NEW = 2 * (segment_off * 512 + off_in_segment) - off_in_segment
+	 */
 	block_off = NAT_BLOCK_OFFSET(start);
-	seg_off = block_off >> sbi->log_blocks_per_seg;
 
 	block_addr = (pgoff_t)(nm_i->nat_blkaddr +
-		(seg_off << sbi->log_blocks_per_seg << 1) +
+		(block_off << 1) -
 		(block_off & (sbi->blocks_per_seg - 1)));
 
 	if (f2fs_test_bit(block_off, nm_i->nat_bitmap))