LU-18173 obdclass: remove lprocfs_strnstr()

LU-18173 obdclass: remove lprocfs_strnstr()

strnstr() was added in the kernel since RHEL6, we no longer need
lprocfs_strnstr() and it can be removed.

Test-Parameters: trivial
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: Id2e3c948dd506b2d983ff776a9136c244d54f6ce
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56166
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>