dump/x86: sanitize the ERESTART_RESTARTBLOCK -> EINTR transition

1. The -ERESTART_RESTARTBLOCK case in get_task_regs() depends on kernel
   internals too much, and for no reason. We shouldn't rely on fact that
   a) we are going to do sigreturn() and b) restore_sigcontext() always
   sets restart_block->fn = do_no_restart_syscall which returns -EINTR.

   Just change this code to enforce -EINTR after restore, this is what
   we actually want until we teach criu to handle ERESTART_RESTARTBLOCK.

2. Add pr_warn() to make the potential bug-reports more understandable,
   a sane application should handle -EINTR correctly but this is not
   always the case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>