LU-18517 quota: fix LC_HAVE_DQUOT_QC_DQBLK check

LU-18517 quota: fix LC_HAVE_DQUOT_QC_DQBLK check

When compiling with clang, the HAVE_DQUOT_QC_DQBLK check always
fails with "error: indirection of non-volatile null pointer will
be deleted, not trap".

Signed-off-by: Sohei Koyama <skoyama@ddn.com>
Change-Id: I8cbdfd70bef8164e40d19d68dc4127c6f40bd693
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57320
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>