xfs: check that bc_nlevels never overflows

Warn if we ever bump nlevels higher than the allowed maximum cursor
height.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>