LU-8066 sysfs: fixup max_dirty_mb

LU-8066 sysfs: fixup max_dirty_mb

Migrate the mdc version of max_dirty_mb to sysfs since its a
simple value visible to non-root users. For the osc version
update the function with sysfs_memparse().

Change-Id: I1329e7a0848101a1754836832e5c3b76d4db76cc
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57457
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>