bpf: reject non-exact register type matches in regsafe()

bpf: reject non-exact register type matches in regsafe()

Generalize the (somewhat implicit) rule of regsafe(), which states that
if register types in old and current states do not match *exactly*, they
can't be safely considered equivalent.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221223054921.958283-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>