mm/cma: add sysfs file 'release_pages_success'

This adds the following new sysfs file tracking the number of successfully
released pages from a given CMA heap area.  This file will be available
via CONFIG_CMA_SYSFS and help in determining active CMA pages available on
the CMA heap area.  This adds a new 'nr_pages_released' (CONFIG_CMA_SYSFS)
into 'struct cma' which gets updated during cma_release().

/sys/kernel/mm/cma/<cma-heap-area>/release_pages_success

After this change, an user will be able to find active CMA pages available
in a given CMA heap area via the following method.

Active pages = alloc_pages_success - release_pages_success

That's valuable information for both software designers, and system admins
as it allows them to tune the number of CMA pages available in the system.
This increases user visibility for allocated CMA area and its
utilization.

Link: https://lkml.kernel.org/r/20240206045731.472759-1-anshuman.khandual@arm.com


Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>