LU-18217 build: Ensure LINUX_RELEASE is defined

LU-18217 build: Ensure LINUX_RELEASE is defined

Ensure LINUX_RELEASE is defined to something consumable by buildrpm

Ex:
  sh ./autogen.sh
  ./configure --enable-dist
  make srpm

Test-Parameters: trivial
Fixes: 0c46ba62efb ("LU-17850 build: prefer LINUXRELEASE over uname -r")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I647a27729c4d14a3983edc74f19a420bb6968631
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56333
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>