xfs: fix __user annotations for xfs_ioc_getfsmap

By passing the whole fsmap_head structure and an index we can get the
user point annotations right for the embedded variable sized array
in struct fsmap_head.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: change idx to unsigned int]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>