CIFS: Add support for direct I/O write

With direct I/O write, user supplied buffers are pinned to the memory and data
are transferred directly from user buffers to the transport layer.

Change in v3: add support for kernel AIO

Change in v4:
Refactor common write code to __cifs_writev for direct and non-direct I/O.
Retry on direct I/O failure.

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>