Linux 3.19: struct nameidata becomes opaque

Linux 3.19: struct nameidata becomes opaque

With kernel 3.19 struct nameidata becomes opaque.  As a result
we cannot rely on STRUCT_NAMEIDATA_HAS_PATH being true for
new kernels.

Rework the conditions here so that STRUCT_NAMEIDATA_HAS_PATH
is only tested when we're using a nameidata structure and
the result matters.

Also modify a configure test to use a nameidata pointer
instead of an actual structure.

Reviewed-on: http://gerrit.openafs.org/11648
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 72e22eb00f641f137f7dbe4195d6d82f4a8addc9)

Change-Id: Ia794d9006a054d16a3b9e5b8ced55c798244d4c7
Reviewed-on: http://gerrit.openafs.org/11662
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>