Dump SigBlk instead of SigCgt

It's because this mask will be restored as rt_sigframe->uc.uc_sigmask.
uc_sigmask will be set as current->blocked in sigreturn, so we should
dump SigBlk in this case.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>