The early check if we should attempt compression does not take into
account the number of input pages. It can happen that there's only one
page, eg. a tail page after some ranges of the BTRFS_MAX_UNCOMPRESSED
have been processed, or an isolated page that won't be converted to an
inline extent.

The single page would be compressed but a later check would drop it
again because the result size must be at least one block shorter than
the input. That can never work with just one page.

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: David Sterba <dsterba@suse.com>