mm/secretmem: use a folio in secretmem_fault()

Saves four implicit call to compound_head().

Link: https://lkml.kernel.org/r/20230812062612.3184990-1-zhangpeng362@huawei.com


Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>