btrfs: use page alloc/free wrappers for compression pages

This is a preparation for managing compression pages in a cache-like
manner, instead of asking the allocator each time. The common allocation
and free wrappers are introduced and are functionally equivalent to the
current code.

The freeing helpers need to be carefully placed where the last reference
is dropped.  This is either after directly allocating (error handling)
or when there are no other users of the pages (after copying the contents).

It's safe to not use the helper and use put_page() that will handle the
reference count. Not using the helper means there's lower number of
pages that could be reused without passing them back to allocator.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>