f2fs: use correct flag in f2fs_map_blocks()

We introduce F2FS_GET_BLOCK_READ in commit e2b4e2bc ("f2fs: fix
incorrect mapping for bmap"), but forget to use this flag in the right
place, fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>