mountfsd: allow O_NONBLOCK on image fds

Let's accept O_NONBLOCK on image fds. It's usually wise to open block
device with O_NONBLOCK as it means that drivers with removable media
won't block until a medium is inserted. Otherwise it has no effect. It
also has no effect when opening regular files. Since block devices and
raw devices are the only two things we accept in mountfsd, it's hence
safe to allow it.

src/mountfsd/mountwork.c

@@ -124,7 +124,7 @@ static int validate_image_fd(int fd, MountImageParameters *p) {
                         return r;
         }
 
-        fl = fd_verify_safe_flags(fd);
+        fl = fd_verify_safe_flags_full(fd, O_NONBLOCK);
         if (fl < 0)
                 return log_debug_errno(fl, "Image file descriptor has unsafe flags set: %m");