f2fs: avoid splitting bio when reading multiple pages

When fewer pages are read, nr_pages may be smaller than nr_cpages. Due
to the nr_vecs limit, the compressed pages will be split into multiple
bios and then merged at the block level. In this case, nr_cpages should
be used to pre-allocate bvecs.
To handle this case, align max_nr_pages to cluster_size, which should be
enough for all compressed pages.

Signed-off-by: Jianan Huang <huangjianan@xiaomi.com>
Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>