xfs: rename bio_add_buffer() to xfs_bio_add_buffer()

Follow up with xfs naming style.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>