The get_node_page_ra tries to:
1. grab or read a target node page for the given nid,
2. then, call ra_node_page to read other adjacent node pages in advance.
So, when we try to read a target node page by #1, we should submit bio with
READ_SYNC instead of READA.
And, in #2, READA should be used.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com> Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>