Linux: use standard macro for set_nlink configure test

Linux: use standard macro for set_nlink configure test

A generic macro exists to test for functions in the kernel, use
it for set_nlink.

Reviewed-on: http://gerrit.openafs.org/6566
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 64bd0b728ca95ba7bb4f1fdd909386fde3ce81e1)

Change-Id: I93d169bec8f476d5e692f7f5a7fe31002af7ce1e
Reviewed-on: http://gerrit.openafs.org/6569
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>