LU-13253 libcfs: protect libcfs_debug_dumplog()

LU-13253 libcfs: protect libcfs_debug_dumplog()

as it uses global state to wait for dumping thread completion and
it doesn't make sense to dump concurrently.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ie6916a0bb53a0a3aa8992413811fe7a908f86276
Reviewed-on: https://review.whamcloud.com/37588
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>