lib/show_mem.c: correct reserved memory calculation

CMA reserved memory is not part of total reserved memory.  Currently
when we print the total reserve memory it considers cma as part of
reserve memory and do minus of totalcma_pages from reserved, which is
wrong.  In cases where total reserved is less than cma reserved we will
get negative values & while printing we print as unsigned and we will
get a very large value.

Below is the show mem output on X86 ubuntu based system where CMA
reserved is 100MB (25600 pages) & total reserved is ~40MB(10316 pages).
And reserve memory shows a large value because of this bug.

Before:
[  127.066430] 898908 pages RAM
[  127.066432] 671682 pages HighMem/MovableOnly
[  127.066434] 4294952012 pages reserved
[  127.066436] 25600 pages cma reserved

After:
[   44.663129] 898908 pages RAM
[   44.663130] 671682 pages HighMem/MovableOnly
[   44.663130] 10316 pages reserved
[   44.663131] 25600 pages cma reserved

Signed-off-by: Vishnu Pratap Singh <vishnu.ps@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Danesh Petigara <dpetigara@broadcom.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>