LU-11986 libcfs: lnet_remove_debugfs() compat for RHEL6

LU-11986 libcfs: lnet_remove_debugfs() compat for RHEL6

Unloading libcfs module on RHEL 6.10 Lustre client with
kernel 2.6.32-754.24.3 hit kernel panic issue. The issue
doesn't exist in Lustre b2_10 where RHEL 6.10 is supported
and debugfs_remove_recursive() is called directly from
lnet_remove_debugfs(). This patch adds compat changes to
lnet_remove_debugfs() to resolve the issue.

Fixes: 9d42660e173e ("LU-11986 lnet: properly cleanup lnet debugfs files")
Fixes: ae93a9f21752 ("LU-11986 libcfs: add compat for d_hash_and_lookup()")
Test-Parameters: trivial
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Change-Id: Ib63a40afe8926f56cd1d2873975855c226098418
Reviewed-on: https://review.whamcloud.com/38716
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>