LU-17592 build: kernel 6.8 -Werror=missing-prototypes

LU-17592 build: kernel 6.8 -Werror=missing-prototypes

Linux commit v6.7-rc4-156-g0fcb70851fbf
  Makefile.extrawarn: turn on missing-prototypes globally

With -Wmissing-prototypes and -Werror cleanup some additional
funtions that are implicitly static and provide declarations
for those that are exported.

Add SERVER_ONLY and SERVER_ONLY_EXPORT_SYMBOL to wrap functions
that are only exported for and used by server components.

Test-Parameters: trivial
HPE-bug-id: LUS-12181
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ice5219df5463effe964d2cd2114f003d185337da
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54228
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>