xfs: consolidate the xfs_quota_reserve_blkres definitions

xfs_trans_reserve_quota_nblks is already stubbed out if quota support
is disabled, no need for an extra xfs_quota_reserve_blkres stub.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>