bpf: mark a NULL memory argument of a call precise

check_mem_reg() allows bpf_register_is_null() for nullable arguments
w/o marking the underlying scalar register precise. Hence a checkpoint
created on such a path would prune against arbitrary scalar value.

The argument may live on the stack rather than in a register when a
call has more than MAX_BPF_FUNC_REG_ARGS arguments, hence the new
mark_arg_precision() helper.

Fixes: e5069b9c23b3 ("bpf: Support pointers in global func args")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-3-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>