bpf: Refactor check_cfg to use a structured loop.

The current implementation uses a number of gotos to implement a loop
and different paths within the loop, which makes the code less readable
than it would be with an explicit while-loop. This patch also replaces a
chain of if/if-elses keyed on the same expression with a switch
statement.

No change in behaviour is intended.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201121015509.3594191-1-wedsonaf@google.com