CVE-2026-72111: bpf: Reset register bounds before narrowing retval range in check_mem_access()

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

bpf: Reset register bounds before narrowing retval range in checkmemaccess()

When the BPF verifier processes a context load of an LSM hook return value, it calls markregs32range() to narrow the register to the hook's valid range. However, markregs32range() intersects the new range with the register's existing bounds using maxt()/mint() rather than replacing them.

If the destination register carries stale bounds from a prior instruction (e.g. BPFMOV64IMM), the intersection can produce a range narrower than reality. The verifier then believes it knows the register's exact value, while at runtime the actual hook return value is loaded, creating a verifier/runtime mismatch that can be used to bypass BPF memory safety checks.

The else branch already calls markregunknown() to reset register state before any narrowing. Apply the same reset in the isretval path so stale bounds are cleared before markregs32range() intersects.