LU-8191 lustre: convert mdc,mdd,mdt,mgc functions to static

LU-8191 lustre: convert mdc,mdd,mdt,mgc functions to static

Static analysis shows that a number of functions
could be made static. This patch declares several
functions in mdc, mdd, mdt, and mgc static.

Also, remove mgs_client_add() since it was unused, and
move a declaration from a c-file to the proper header file.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ia23f62465c27c83a9a0260bb45e8c8b710491558
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51478
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>