xfs: check for obviously bad level values in the bmbt root

xfs: check for obviously bad level values in the bmbt root

BugLink: http://bugs.launchpad.net/bugs/1681875
commit b3bf607d58520ea8c0666aeb4be60dbb724cd3a2 upstream.

We can't handle a bmbt that's taller than BTREE_MAXLEVELS, and there's
no such thing as a zero-level bmbt (for that we have extents format),
so if we see this, send back an error code.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>