xfs: simplify xfs_map_blocks by using xfs_iext_lookup_extent directly

xfs_bmapi_read adds zero value in xfs_map_blocks.  Replace it with a
direct call to the low-level extent lookup function.

Note that we now always pass a 0 length to the trace points as we ask
for an unspecified len.

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>