LU-18986 mgs: server part of new registration protocol

LU-18986 mgs: server part of new registration protocol

Rework mgs_target_reg() to handle new protocol along with
old one for older targets

It handles old protocol with NIDs either in mti_nids or
in mti_nidlist[], and new protocol with NIDs in
mtn_inline_list[] or bulk

All NIDs are put in mti_nidlist[] as result of request
processing, so that eliminates need in extra changes in
further code path

Test-Parameters: testlist=runtests ossversion=EXA6.3.2
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I41dd487c37136e24328914e33c9ce056be013aae
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59206
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>