target: Rename core_tpg_{pre,post}_addlun for clarity

"pre" is really an allocation function. The only time it isn't called is
for virtual_lun0, which is statically allocated. Renaming that to "alloc"
lets the other function not need to be "post", and just be called
core_tpg_add_lun.

(nab: fix minor applying fuzz in core_tpg_setup_virtual_lun0)

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