LU-13805 llite: add flag to disable unaligned DIO

LU-13805 llite: add flag to disable unaligned DIO

As with any new IO feature, it's a good idea to have the
option to turn off unaligned DIO support if needed.

It would be reasonable to merge this patch with the core
patch implementing the feature; I have kept it separate for
ease of review.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ibc86d84704151a7f30afcc538d9c03e3fdf1c38a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51125
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>