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>