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Currently we have two wrappers to allocate and free a page for
compression usage:

- btrfs_alloc_compr_page()
- btrfs_free_compr_page()

The allocator would try to grab a page from the pool, and only allocate
a new page if the pool is empty.

The reclaimer would check if the pool is full, and if not full it would
put the page into the pool.

This patch converts both helpers to use folio interfaces, and allowing
further conversion of compression path to folios.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>