xfs: Clear W=1 warning in xfs_iwalk_run_callbacks()

For CONFIG_XFS_DEBUG unset, xfs_iwalk_run_callbacks() generates the
following warning for when building with W=1:

fs/xfs/xfs_iwalk.c: In function ‘xfs_iwalk_run_callbacks’:
fs/xfs/xfs_iwalk.c:354:42: error: variable ‘irec’ set but not used [-Werror=unused-but-set-variable]
  354 |         struct xfs_inobt_rec_incore     *irec;
      |                                          ^~~~
cc1: all warnings being treated as errors

Drop @irec, as it is only an intermediate variable.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>