xfs: ensure xfs_errortag_random_default matches XFS_ERRTAG_MAX

Add the BUILD_BUG_ON to xfs_errortag_add() in order to make sure that
the length of xfs_errortag_random_default matches XFS_ERRTAG_MAX when
building.

Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>