CVE-2026-64369: s390: Revert support for DCACHE_WORD_ACCESS

In the Linux kernel, the following vulnerability has been resolved:

s390: Revert support for DCACHEWORDACCESS

loadunalignedzeropad() reads eight bytes from unaligned addresses and may cross page boundaries. It handles exceptions which may happen if reading from the second page results in an exception.

For pages which are donated to the Ultravisor for secure execution purposes the dosecurestorageaccess() exception handler however does not handle such exceptions correctly. Such an exception may result in an endless exception loop which will never be resolved.

An attempt to fix this [1] turned out to be not sufficient. For now revert loadunalignedzeropad() until this problem has been resolved in a proper way.

Note that the implementation of loadunalignedzeropad() itself is correct. The revert is just a temporary workaround until there is complete fix for secure storage access exceptions.

[1] commit b00be77302d7 ("s390/mm: Add missing secure storage access fixups for donated memory")