f2fs: fix cgroup writeback accounting with fs-layer encryption

When writing a page from an encrypted file that is using
filesystem-layer encryption (not inline encryption), f2fs encrypts the
pagecache page into a bounce page, then writes the bounce page.

It also passes the bounce page to wbc_account_cgroup_owner().  That's
incorrect, because the bounce page is a newly allocated temporary page
that doesn't have the memory cgroup of the original pagecache page.
This makes wbc_account_cgroup_owner() not account the I/O to the owner
of the pagecache page as it should.

Fix this by always passing the pagecache page to
wbc_account_cgroup_owner().

Fixes: 578c6478 ("f2fs: implement cgroup writeback support")
Cc: stable@vger.kernel.org
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>