LU-17030 llite: allow setting max_cached_mb to a %

LU-17030 llite: allow setting max_cached_mb to a %

Lustre's max_cached_mb parameter is hard to use because it
must be set to a specific numeric value, so in effect it
cannot be set on the server side unless all clients are
guaranteed identical.

Let's add the ability to set that to a % of memory to make
it more useful.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I1f9f5a8a5d671ab00b7ab6133bb9b1d1214ca59e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51952
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>