CVE-2026-99158: ring-buffer: Fix false warning in ring_buffer_map_get_reader()

In the Linux kernel, the following vulnerability has been resolved:

ring-buffer: Fix false warning in ringbuffermapgetreader()

The mmap reader can warn when it catches up with a writer that is still committing events. rbgetreaderpage() returns NULL in this case, but ringbuffermapgetreader() treats that result as an error.

The initial rbpercpuempty() check filters out an empty buffer, but it does not guarantee that the next reader page has committed data. Writers do not take readerlock and can advance commitpage before publishing the committed length on the new page. rbgetreaderpage() can swap to that page, catch up with the writer and return NULL.

Handle NULL through the existing no-data exit without warning. Remove the caller's readerpage == commitpage check as well, since the reader-page helper already handles that case. The existing metadata update and return path remain in place.