arm64/sme: Always exit sme_alloc() early with existing storage

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

arm64/sme: Always exit sme_alloc() early with existing storage

When sme_alloc() is called with existing storage and we are not flushing we

will always allocate new storage, both leaking the existing storage and

corrupting the state. Fix this by separating the checks for flushing and

for existing storage as we do for SVE.

Callers that reallocate (eg, due to changing the vector length) should

call sme_free() themselves.