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: 33a8f7f2b3a3 ("pagemap,pmem: introduce ->memory_failure()")
Signed-off-by: Dan Williams <[email protected]>
Acked-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Shiyang Ruan <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Goldwyn Rodrigues <[email protected]>
Cc: Jane Chu <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Ritesh Harjani <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions