target: Cleanup transport_kunmap_data_sg()

This code isn't broken per se, but it's scary to look at! It looks like
in the t_data_nents==1 case we're doing both a kunmap and a vunmap,
what's saving us is that t_data_vmap in this case is 0, so vunmap
doesn't do anything.

Return after kunmap, so the handling of the three cases does not overlap.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>