If we can't read the AGF header, we never actually set a value for
freelen and usedlen.  These two variables are used to make the worst
case estimate of btree size, so it's safe to set them to the AG size as
a fallback.
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NChristoph Hellwig <hch@lst.de>