LU-11986 libcfs: add compat for d_hash_and_lookup()

Add compatibility for older kernels that do not export the
d_hash_and_lookup() function, even though it exists forever.

Fixes: 9d42660e173e ("LU-11986 lnet: properly cleanup lnet debugfs files")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I10d6547f6b17665880cacfbf87d4dbfd386b92ff
Reviewed-on: https://review.whamcloud.com/38529
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>