bpf: simplify __is_valid_access test on cb

The __is_valid_access() test for cb[] from 62c7989b ("bpf: allow
b/h/w/dw access for bpf's cb in ctx") was done unnecessarily complex,
we can just simplify it the same way as recent fix from 2d071c64
("bpf, trace: make ctx access checks more robust") did. Overflow can
never happen as size is 1/2/4/8 depending on access.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>