bpf: move explored_state() closer to the beginning of verifier.c

Subsequent patches would make use of explored_state() function.
Move it up to avoid adding unnecessary prototype.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231024000917.12153-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>