f2fs: avoid latency-critical readahead of node pages

The f2fs_map_blocks is very related to the performance, so let's avoid any
latency to read ahead node pages.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -652,7 +652,7 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 	unsigned int maxblocks = map->m_len;
 	struct dnode_of_data dn;
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
-	int mode = create ? ALLOC_NODE : LOOKUP_NODE_RA;
+	int mode = create ? ALLOC_NODE : LOOKUP_NODE;
 	pgoff_t pgofs, end_offset, end;
 	int err = 0, ofs = 1;
 	unsigned int ofs_in_node, last_ofs_in_node;

fs/f2fs/file.c

@@ -354,7 +354,7 @@ static loff_t f2fs_seek_block(struct file *file, loff_t offset, int whence)
 
 	for (; data_ofs < isize; data_ofs = (loff_t)pgofs << PAGE_SHIFT) {
 		set_new_dnode(&dn, inode, NULL, NULL, 0);
-		err = get_dnode_of_data(&dn, pgofs, LOOKUP_NODE_RA);
+		err = get_dnode_of_data(&dn, pgofs, LOOKUP_NODE);
 		if (err && err != -ENOENT) {
 			goto fail;
 		} else if (err == -ENOENT) {