xfs: refactor xfs_bmap_count_blocks using newer btree helpers

Currently, this function open-codes walking a bmbt to count the extents
and blocks in use by a particular inode fork.  Since we now have a
function to tally extent records from the incore extent tree and a btree
helper to count every block in a btree, replace all that with calls to
the helpers.

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