xfs: simplify the read/write tracepoints

Pass the iocb and iov_iter to the tracepoints and leave decoding of
actual arguments to the code only run when tracing is enabled.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>