f2fs: introduce mempool for {,de}compress intermediate page allocation

If compression feature is on, in scenario of no enough free memory,
page refault ratio is higher than before, the root cause is:
- {,de}compression flow needs to allocate intermediate pages to store
compressed data in cluster, so during their allocation, vm may reclaim
mmaped pages.
- if above reclaimed pages belong to compressed cluster, during its
refault, it may cause more intermediate pages allocation, result in
reclaiming more mmaped pages.

So this patch introduces a mempool for intermediate page allocation,
in order to avoid high refault ratio, by default, number of
preallocated page in pool is 512, user can change the number by
assigning 'num_compress_pages' parameter during module initialization.

Ma Feng found warnings in the original patch and fixed like below.

Fix the following sparse warning:
fs/f2fs/compress.c:501:5: warning: symbol 'num_compress_pages' was not declared.
 Should it be static?
fs/f2fs/compress.c:530:6: warning: symbol 'f2fs_compress_free_page' was not
declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -3879,7 +3879,12 @@ static int __init init_f2fs_fs(void)
 	err = f2fs_init_bioset();
 	if (err)
 		goto free_bio_enrty_cache;
+	err = f2fs_init_compress_mempool();
+	if (err)
+		goto free_bioset;
 	return 0;
+free_bioset:
+	f2fs_destroy_bioset();
 free_bio_enrty_cache:
 	f2fs_destroy_bio_entry_cache();
 free_post_read:
@@ -3907,6 +3912,7 @@ fail:
 
 static void __exit exit_f2fs_fs(void)
 {
+	f2fs_destroy_compress_mempool();
 	f2fs_destroy_bioset();
 	f2fs_destroy_bio_entry_cache();
 	f2fs_destroy_post_read_processing();