cifs: Fix smb3_insert_range() to move the zero_point

Fix smb3_insert_range() to move the zero_point over to the new EOF.
Without this, generic/147 fails as reads of data beyond the old EOF point
return zeroes.

Fixes: 3ee1a1fc ("cifs: Cut over to using netfslib")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
Signed-off-by: Steve French <stfrench@microsoft.com>