bpf_jit_disasm: also support reading jit dump from file

This patch adds support to read the dmesg BPF JIT dump also from a
file instead of the klog buffer. I found this quite useful when going
through some 'before/after patch' logs. It also fixes a regex leak
found by valgrind when no image dump was found.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>