Simplify allocation check in storageVolResize
Since shrinking a volume below existing allocation is not allowed,
it is not possible for a successful resize with VOL_RESIZE_ALLOCATE
to increase the pool's available value.
Even with the SHRINK flag it is possible to extend the current
allocation or even the capacity. Remove the overflow when
computing delta with this flag and do the check even if the
flag was specified.
https://bugzilla.redhat.com/show_bug.cgi?id=1073305