sparc: Fix handling of page table constructor failure

The page has just been allocated, so its refcount is 1.  free_unref_page()
is for use on pages which have a zero refcount.  Use __free_page()
like the other implementations of pte_alloc_one().

Fixes: 1ae9ae5f

 ("sparc: handle pgtable_page_ctor() fail")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>