virt: sev-guest: Do not use host-controlled page order in cleanup path

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

virt: sev-guest: Do not use host-controlled page order in cleanup path

When issuing an extended guest request (SVM_VMGEXIT_EXT_GUEST_REQUEST),

get_ext_report() allocates a buffer to retrieve a certificate blob from the

host, keeping track of its size in report_req->certs_len.

However, the host may return SNP_GUEST_VMM_ERR_INVALID_LEN, indicating

an invalid buffer size, as well as the expected length of such buffer.

get_ext_report() subsequently updates report_req->certs_len with the

host-controlled value, and cleans up the buffer by computing a page order

from such value. This is incorrect, as the host-provided length may not

match the page order of the original allocation, potentially resulting

in corruption in the page allocator.

Fix this by using alloc_pages_exact() instead, and reusing @npages to

compute the size passed to free_pages_exact(). For consistency, also

use @npages to compute the size when allocating the pages, even though

this last change has no functional effect.