mm: use alloc_pages_exact() in alloc_large_system_hash() to avoid duplicated logic

alloc_large_system_hash() has logic for freeing pages at the end of an
excessively large power-of-two buffer that is a duplicate of what is in
alloc_pages_exact().  This patch converts alloc_large_system_hash() to use
alloc_pages_exact().

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>