f2fs: Use folio in f2fs_read_merkle_tree_page

Use folio in f2fs_read_merkle_tree_page to reduce folio & page converisons
from find_get_page_flags and read_mapping_page functions. But the return
value should be the exact page.

Signed-off-by: HuangXiaojia <huangxiaojia2@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>