LU-13465 mgs: re-introduce mgs_set_info for interop purpose

LU-13465 mgs: re-introduce mgs_set_info for interop purpose

Older clients might send MGS_SET_INFO requests to the MGS. Newer MGS
needs to handle that gracefully by returning:
- -EINVAL for anything other than lov.stripe*
- 0 for those parameters without doing anything, rather than
  returning -ENOTSUPP.

Test-Parameters: clientversion=2.12 envdefinitions=SANITY_EXCEPT="27M 56ra 151 156 802"
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iba75c788eaefabd98bbb65a657f9a0150e20fafb
Reviewed-on: https://review.whamcloud.com/38311
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>