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.

Change-Id: I0d71fca44a67570ac3b86151c70f2969dc463f4f
Reviewed-on: http://gerrit.openafs.org/11648
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>