ring-buffer: Remove unused function ring_buffer_nr_pages()

Because ring_buffer_nr_pages() is not an inline function and user accesses
buffer->buffers[cpu]->nr_pages directly, the function ring_buffer_nr_pages
is removed.

Signed-off-by: Jianhui Zhou <912460177@qq.com>
Link: https://lore.kernel.org/tencent_F4A7E9AB337F44E0F4B858D07D19EF460708@qq.com


Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>