mm/gup: Let __get_user_pages_locked() return -EINTR for fatal signal

__get_user_pages_locked() will return 0 instead of -EINTR after commit
4426e945df588 ("mm/gup: allow VM_FAULT_RETRY for multiple times") which
added extra code to allow gup detect fatal signal faster.

Restore the original -EINTR behavior.

Cc: Andrew Morton <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Fixes: 4426e945df58 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
Reported-by: [e-mail]
Signed-off-by: Hillf Danton <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Signed-off-by: Peter Xu <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/gup.c

···
 		 * start trying again otherwise it can loop forever.
 		 */
 
-		if (fatal_signal_pending(current))
+		if (fatal_signal_pending(current)) {
+			if (!pages_done)
+				pages_done = -EINTR;
 			break;
+		}
 
 		ret = down_read_killable(&mm->mmap_sem);
 		if (ret) {