mm: do not allocate page->ptl dynamically, if spinlock_t fits to long

mm: do not allocate page->ptl dynamically, if spinlock_t fits to long

In struct page we have enough space to fit long-size page->ptl there,
but we use dynamically-allocated page->ptl if size(spinlock_t) is larger
than sizeof(int).

It hurts 64-bit architectures with CONFIG_GENERIC_LOCKBREAK, where
sizeof(spinlock_t) == 8, but it easily fits into struct page.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>