Remove the macro XFS_BUF_SET_TARGET.

hch: As all the buffer allocator already set ->b_target it should be safe
to simply remove these calls.
Signed-off-by: NChandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAlex Elder <aelder@sgi.com>