bpf-devices: normalize how we pass around major/minor values

There's some unclarity whether major/minor of device nodes are supposed
to be "unsigned" or "dev_t". Various codebases assume the latter, but
glibc's major()/minor() types actually return a value typed to
"unsigned". On glibc dev_t is actually 64bit even if the kernel only
exposes 32bit. Hence this distinction kinda matters.

Let's clean things up a bit with handling: let's followe glibc's type
system here, and use unsigned (and not int).

Also let's pass invalid major/minor values around as UINT_MAX rather
than via pointers, to match how we usually do this, and to shorten our
code a bit. This is safe, since given the linux dev_t space being 32bit
only we can't possibly have a valid major or minor this hight, given
they must be smaller in size. While other archs disagree on the types of
major/minor, they also tend to have similar limits. In fact on FreeBSD
for example major()/minor() returns a signed int. Which would hence also
mean that UINT_MAX cannot be a valid major or minor.

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