Search for the Linux version in the correct header

Search for the Linux version in the correct header

src/config/linux-version already had support for handling either versioned
or unversioned kernel source directories, but the grep for UTS_RELEASE
was unconditionally done against the versioned source tree, ignoring the
previous determination of where the header file was.

Instead, check instead the header file location that we'd previously
determined.

This patch has been tested in the Debian OpenAFS 1.4.11~pre3+dfsg-1
package.

Reviewed-on: http://gerrit.openafs.org/4
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Verified-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>