CAS-14622: use NoLocking in writeBackToFullImage for tile-aligned images
Switch PagedImage open mode from AutoNoReadLocking to NoLocking in
CubeMajorCycleAlgorithm::writeBackToFullImage.
This is safe because the tile-alignment fix in SIImageStore::buildImage
(also in this branch) guarantees that each MPI worker's channel range
maps to a disjoint set of TiledStMan disk tiles. With no physically
shared pages between workers, the write-lock cycle that AutoNoReadLocking
imposes on every flush (request → wait → acquire → flush → release) adds
unnecessary Lustre lock overhead even though the underlying I/O regions
never overlap.
