posix: eventfd: Fix unsetting internal flags in ioctl

Commit e6eb0a7 ("posix: eventfd: revise locking, signaling, and
allocation") introduced a regression where the internal flags of an
event file descriptor would be erased when calling the F_SETFL ioctl
operation.

This includes the flag EFD_IN_USE_INTERNAL which determines whether
this file descriptor has been opened, thus effectively closing the
eventfd whenever one tries to change a flag.

Signed-off-by: Celina Sophie Kalus <[email protected]>
(cherry picked from commit 5bd86ea)

lib/posix/eventfd.c

@@ -247,7 +247,9 @@ static int eventfd_ioctl_op(void *obj, unsigned int request, va_list args)
 			errno = EINVAL;
 			ret = -1;
 		} else {
-			efd->flags = flags;
+			int prev_flags = efd->flags & ~EFD_FLAGS_SET_INTERNAL;
+
+			efd->flags = flags | prev_flags;
 			ret = 0;
 		}
 	} break;