Merge branch 'bpf-make-reg_not_null-true-for-const_ptr_to_map'

Ihor Solodrai says:

====================
bpf: make reg_not_null() true for CONST_PTR_TO_MAP

Handle CONST_PTR_TO_MAP null checks in the BPF verifier. Add
appropriate test cases.

v3->v4: more test cases
v2->v3: change constant in unpriv test
v1->v2: add a test case with ringbufs

v3: https://lore.kernel.org/bpf/20250604222729.3351946-1-isolodrai@meta.com/
v2: https://lore.kernel.org/bpf/20250604003759.1020745-1-isolodrai@meta.com/
v1: https://lore.kernel.org/bpf/20250523232503.1086319-1-isolodrai@meta.com/
====================

Link: https://patch.msgid.link/20250609183024.359974-1-isolodrai@meta.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>