xfs: assert that xfs_reflink_allocate_cow is called with XFS_ILOCK_EXCL

Now that we convert COW preallocations from unwritten to real on every
call this function needs to be called with the ilock held exclusively.

Fortunately we already do that, but update the assert to match.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>