seccomp: Fix nits in collect_filter

- Fix typo in sizeof() operand
 - Eliminate redundant prctl calls if no PTRACE_SECCOMP_GET_METADATA
   detected

Reported-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>