Btrfs: fix eb memory leak due to readpage failure

eb->io_pages is set in read_extent_buffer_pages().

In case of readpage failure, for pages that have been added to bio,
it calls bio_endio and later readpage_io_failed_hook() does the work.

When this eb's page (couldn't be the 1st page) fails to add itself to bio
due to failure in merge_bio(), it cannot decrease eb->io_pages via bio_endio,
 and ends up with a memory leak eventually.

This lets __do_readpage propagate errors to callers and adds the
 'atomic_dec(&eb->io_pages)'.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>