MEMORY calculate small allocation size dynamically

rather than hardcoding the size calculate it dynamically
at iscsi_create_context().

Signed-off-by: Peter Lieven <pl@kamp.de>
changed iscsi_data.size from int to size_t

iscsi_data.size is used as parameter to memory
operation functions (such as malloc) which except
size_t rather than int.

Signed-off-by: Peter Lieven <pl@kamp.de>