mm: thp: microoptimize compound_mapcount()

compound_mapcount() is only called after PageCompound() has already been
checked by the caller, so there's no point to check it again.  Gcc may
optimize it away too because it's inline but this will remove the
runtime check for sure and add it'll add an assert instead.

Link: http://lkml.kernel.org/r/1462547040-1737-3-git-send-email-aarcange@redhat.com


Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>