CVE-2026-80537: In the Linux kernel, the following vulnerability has been resolved: xfs: fix off-by-one in rtrefcount btree root level

In the Linux kernel, the following vulnerability has been resolved:

xfs: fix off-by-one in rtrefcount btree root level validation

xfs_rtrefcountbt_compute_maxlevels() sets

mp->m_rtrefc_maxlevels = min(d_maxlevels, r_maxlevels) + 1;

where the trailing "+ 1" already accounts for the inode-root level, so the deepest valid on-disk root level is m_rtrefc_maxlevels - 1 and a cursor must satisfy bc_nlevels