xfs: fix integer truncation in xfs_bmap_remap_alloc

bno should be a xfs_fsblock_t, which is 64-bit wides instead of a
xfs_aglock_t, which truncates the value to 32 bits.

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>