ring-buffer: Do not warn in ring_buffer_map_get_reader() when reader catches up

In the Linux kernel, the following vulnerability has been resolved: ring-buffer: Do not warn in ring_buffer_map_get_reader() when reader catches up The function ring_buffer_map_get_reader() is a bit more strict than the other get reader functions, and except for certain situations the rb_get_reader_page() should not return NULL. If it does, it triggers a warning. This warning was triggering but after looking at why, it was because another acceptable situation was happening and it wasn't checked for. If the reader catches up to the writer and there's still data to be read on the reader page, then the rb_get_reader_page() will return NULL as there's no new page to get. In this situation, the reader page should not be updated and no warning should trigger.