mm/memory-failure: fix detection of memory_failure() handlers

Some pagemap types, like MEMORY_DEVICE_GENERIC (device-dax) do not even
have pagemap ops which results in crash signatures like this:

  BUG: kernel NULL pointer dereference, address: 0000000000000010
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 8000000205073067 P4D 8000000205073067 PUD 2062b3067 PMD 0
  Oops: 0000 [#1] PREEMPT SMP PTI
  CPU: 22 PID: 4535 Comm: device-dax Tainted: G           OE    N 6.0.0-rc2+ #59
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:memory_failure+0x667/0xba0
 [..]
  Call Trace:
   <TASK>
   ? _printk+0x58/0x73
   do_madvise.part.0.cold+0xaf/0xc5

Check for ops before checking if the ops have a memory_failure()
handler.

Link: https://lkml.kernel.org/r/166153428781.2758201.1990616683438224741.stgit@dwillia2-xfh.jf.intel.com
Fixes: 33a8f7f ("pagemap,pmem: introduce ->memory_failure()")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/memremap.h

@@ -139,6 +139,11 @@ struct dev_pagemap {
 	};
 };
 
+static inline bool pgmap_has_memory_failure(struct dev_pagemap *pgmap)
+{
+	return pgmap->ops && pgmap->ops->memory_failure;
+}
+
 static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
 {
 	if (pgmap->flags & PGMAP_ALTMAP_VALID)

mm/memory-failure.c

@@ -1928,7 +1928,7 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
 	 * Call driver's implementation to handle the memory failure, otherwise
 	 * fall back to generic handler.
 	 */
-	if (pgmap->ops->memory_failure) {
+	if (pgmap_has_memory_failure(pgmap)) {
 		rc = pgmap->ops->memory_failure(pgmap, pfn, 1, flags);
 		/*
 		 * Fall back to generic handler too if operation is not