Linux: 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 ring_buffer_map_get_reader()

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

The initial rb_per_cpu_empty() check filters out an empty buffer, but it does not guarantee that the next reader page has committed data. Writers do not take reader_lock and can advance commit_page before publishing the committed length on the new page. rb_get_reader_page() 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 reader_page == commit_page check as well, since the reader-page helper already handles that case. The existing metadata update and return path remain in place.