ptrace: Skip GETREGS on exits from syscalls when possible

The PTRACE_SYSCALL traps task twice -- first on enter into
and then on exit from syscall. If we trace a single task (and
we do it on dump two times per task) we may skip half of all
getregs calls -- on exit we don't need them.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>