LU-13805 llite: add mm to dio struct

LU-13805 llite: add mm to dio struct

When copying to or from userspace, we must use the mm from
the userspace thread.  This can be done either by running
in that thread or borrowing its mm.  Unaligned DIO does
some memory movement to userspace in ptlrpcd threads, so it
requires the user mm be stored in the sub dio.

This will be used by the main unaligned DIO patch and has
been split out for reviewability.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I419cb9f1899b8c8f9790ce25b3aba1d6f07397aa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49947
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>