mm: numa: take THP into account when migrating pages for NUMA balancing

Wanpeng Li pointed out that numamigrate_isolate_page() assumes that only
one base page is being migrated when in fact it can also be checking
THP.

The consequences are that a migration will be attempted when a target
node is nearly full and fail later.  It's unlikely to be user-visible
but it should be fixed.  While we are there, migrate_balanced_pgdat()
should treat nr_migrate_pages as an unsigned long as it is treated as a
watermark.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Suggested-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Simon Jeons <simon.jeons@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>