xfs: make use of the l_targ field in struct xlog

Use the slightly shorter way to get at the buftarg for the log device
wherever we can in the log and log recovery code.

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