slab: do not call compound_head() in page_get_cache()

page_get_cache() does not need to call compound_head(), as its unique
caller virt_to_slab() already makes sure to return a head page.

Additionally, removing the compound_head() call makes page_get_cache()
consistent with page_get_slab().

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>