smb: client: fix (remove) drop_dir_cache module parameter

smb: client: fix (remove) drop_dir_cache module parameter

Being a module parameter, it's possible to do:

  # modprobe cifs drop_dir_cache=1

Which will lead to a crash, because cifs_tcp_ses_list hasn't been
initialized yet:

  [  168.242624] BUG: kernel NULL pointer dereference, address: 0000000000000010
  [  168.242952] #PF: supervisor read access in kernel mode
  [  168.243175] #PF: error_code(0x0000) - not-present page
  [  168.243394] PGD 0 P4D 0
  [  168.243524] Oops: Oops: 0000 [#1] SMP NOPTI
  [  168.243703] CPU: 2 UID: 0 PID: 1105 Comm: modprobe Not tainted 7.0.0-lku #5 PREEMPT(lazy)
  [  168.244054] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-2-g4f253b9b-prebuilt.qemu.org 04/01/2014
  [  168.244557] RIP: 0010:cifs_param_set_drop_dir_cache+0x7c/0x100 [cifs]
  ...
  [  168.248785] Call Trace:
  [  168.248915]  <TASK>
  [  168.249023]  parse_args+0x285/0x3a0
  [  168.249204]  ? __pfx_unknown_module_param_cb+0x10/0x10
  [  168.249448]  load_module+0x192b/0x1bb0
  [  168.249637]  ? __pfx_unknown_module_param_cb+0x10/0x10
  [  168.249882]  ? kernel_read_file+0x27d/0x2b0
  [  168.250088]  init_module_from_file+0xce/0xf0
  [  168.250291]  idempotent_init_module+0xfb/0x2f0
  [  168.250496]  __x64_sys_finit_module+0x5a/0xa0
  [  168.250694]  do_syscall_64+0xe0/0x5a0
  [  168.250863]  ? exc_page_fault+0x65/0x160
  [  168.251050]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  [  168.251284] RIP: 0033:0x7fcaa12b774d

Instead of fixing this with some kind of "is module initialized"
approach, this patch instead moves that functionality to procfs,
setting a write op for the existing open_dirs entry, where
writing a 0 to it will drop the cached directory entries.

Also make it available only when CONFIG_CIFS_DEBUG=y.

A small change needed now is to not call flush_delayed_work()
on invalidate_all_cached_dirs() when called from procfs (can't sleep in
that context).
So add a @sync arg to invalidate_all_cached_dirs() to control when to
flush the delayed works.

Fixes: dde6667fa3c8 ("smb: client: add drop_dir_cache module parameter to invalidate cached dirents")
Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>