netfs: Call `invalidate_cache` only if implemented
In the Linux kernel, the following vulnerability has been resolved:

netfs: Call `invalidate_cache` only if implemented

Many filesystems such as NFS and Ceph do not implement the
`invalidate_cache` method.  On those filesystems, if writing to the
cache (`NETFS_WRITE_TO_CACHE`) fails for some reason, the kernel
crashes like this:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor instruction fetch in kernel mode
 #PF: error_code(0x0010) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0010 [#1] SMP PTI
 CPU: 9 UID: 0 PID: 3380 Comm: kworker/u193:11 Not tainted 6.13.3-cm4all1-hp #437
 Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 10/17/2018
 Workqueue: events_unbound netfs_write_collection_worker
 RIP: 0010:0x0
 Code: Unable to access opcode bytes at 0xffffffffffffffd6.
 RSP: 0018:ffff9b86e2ca7dc0 EFLAGS: 00010202
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 7fffffffffffffff
 RDX: 0000000000000001 RSI: ffff89259d576a18 RDI: ffff89259d576900
 RBP: ffff89259d5769b0 R08: ffff9b86e2ca7d28 R09: 0000000000000002
 R10: ffff89258ceaca80 R11: 0000000000000001 R12: 0000000000000020
 R13: ffff893d158b9338 R14: ffff89259d576900 R15: ffff89259d5769b0
 FS:  0000000000000000(0000) GS:ffff893c9fa40000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: ffffffffffffffd6 CR3: 000000054442e003 CR4: 00000000001706f0
 Call Trace:
  <TASK>
  ? __die+0x1f/0x60
  ? page_fault_oops+0x15c/0x460
  ? try_to_wake_up+0x2d2/0x530
  ? exc_page_fault+0x5e/0x100
  ? asm_exc_page_fault+0x22/0x30
  netfs_write_collection_worker+0xe9f/0x12b0
  ? xs_poll_check_readable+0x3f/0x80
  ? xs_stream_data_receive_workfn+0x8d/0x110
  process_one_work+0x134/0x2d0
  worker_thread+0x299/0x3a0
  ? __pfx_worker_thread+0x10/0x10
  kthread+0xba/0xe0
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x30/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1a/0x30
  </TASK>
 Modules linked in:
 CR2: 0000000000000000

This patch adds the missing `NULL` check.