nfp: bpf: reuse verifier log for debug messages

Now that `bpf_verifier_log_write()` is exported from the verifier and
makes it possible to reuse the verifier log to print messages to the
standard output, use this instead of the kernel logs in the nfp driver
for printing error messages occurring at verification time.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>