ftrace: Allocate the mcount record pages as groups

Allocate the mcount record pages as a group of pages as big
as can be allocated and waste no more than a single page.

Grouping the mcount pages as much as possible helps with cache
locality, as we do not need to redirect with descriptors as we
cross from page to page. It also allows us to do more with the
records later on (sort them with bigger benefits).

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