xfs: don't zap bmbt forks if they are MAXLEVELS tall

LOLLM noticed a discrepancy between the bmbt level checks in the libxfs
bmbt code vs. the inode repair code.  We do actually allow a bmbt root
that proclaims to have a height of XFS_BM_MAXLEVELS.

Cc: stable@vger.kernel.org # v6.8
Fixes: e744cef2060559 ("xfs: zap broken inode forks")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>