binfmt_misc: Make debug printed using pr_debug()

It's a debug message, and it shouldn't be printed with
pr_perror(). Also, we interested in debug in the both
cases: magic and extensions.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>