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>
(cherry picked from commit ecd3fc42d1d6dca23bb2e2728a0066d3db8eceb6)
Change-Id: I286109d139320f7ce71c5d5265bc3e4a2fca4945
Reviewed-on: http://gerrit.openafs.org/924
Tested-by: Derrick Brashear <shadow@dementia.org>