HAVE_RESTORE_SIGMASK is defined on all architectures now

Everyone either defines it in arch thread_info.h or has TIF_RESTORE_SIGMASK
and picks default set_restore_sigmask() in linux/thread_info.h.  Kill the
ifdefs, slap #error in linux/thread_info.h to catch breakage when new ones
get merged.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/eventpoll.c

@@ -1853,8 +1853,6 @@ error_return:
 	return error;
 }
 
-#ifdef HAVE_SET_RESTORE_SIGMASK
-
 /*
  * Implement the event wait interface for the eventpoll file. It is the kernel
  * part of the user space epoll_pwait(2).
@@ -1899,8 +1897,6 @@ SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
 	return error;
 }
 
-#endif /* HAVE_SET_RESTORE_SIGMASK */
-
 static int __init eventpoll_init(void)
 {
 	struct sysinfo si;