smb3: fix perf regression with cached writes with netfs conversion

Write through mode is for cache=none, not for default (when
caching is allowed if we have a lease). Some tests were running
much, much more slowly as a result of disabling caching of
writes by default.

Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Fixes: 3ee1a1fc

 ("cifs: Cut over to using netfslib")
Signed-off-by: Steve French <stfrench@microsoft.com>