btrfs: fix wrong sizeof in btrfs_do_encoded_write()

btrfs_do_encoded_write() was converted to use folios in 400b172b,
but we're still allocating based on sizeof(struct page *) rather than
sizeof(struct folio *). There's no functional change.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>