ANDROID: ring-buffer: Fix rb_page_desc() fast-path

The fast-path for the proper CPU in rb_page_desc() was wrong due to a
missing cast. In practice, we only have trace descriptors with the same
number of pages per CPU, but nonetheless the structure doesn't really
enforce it. Proper support would need a MAGIC in the header of each CPU
as well as a complete struct size in the global header. We do not
anticipate any gain here from this optimization in a first place, so
let's get rid of it entirely.

Bug: 357781595
Fixes: 162c11d2 ("FROMLIST: ring-buffer: Introducing ring-buffer writer")
Change-Id: Idf226cc96e4d1cf5ed959f87c4612f8a24d68cd0
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>