xfs: simplify the xfs_getbmap interface

Instead of passing in a formatter callback allocate the bmap buffer
in the caller and process the entries there.  Additionally replace
the in-kernel buffer with a new much smaller structure, and unify
the implementation of the different ioctls in a single function.

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