xfs: simplify the xfs_getbmap interface

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 <[email protected]>
Reviewed-by: Brian Foster <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>