KVM: SEV: Use kvzalloc_objs() when pinning userpages

Use kvzalloc_objs() instead of sev_pin_memory()'s open coded (rough)
equivalent to harden the code and

Note! This sanity check in __kvmalloc_node_noprof()

/* Don't even allow crazy sizes */
if (unlikely(size > INT_MAX)) {
WARN_ON_ONCE(!(flags & __GFP_NOWARN));
return NULL;
}

will artificially limit the maximum size of any single pinned region to
just under 1TiB. While there do appear to be providers that support SEV
VMs with more than 1TiB of _total_ memory, it's unlikely any KVM-based
providers pin 1TiB in a single request.

Allocate with NOWARN so that fuzzers can't trip the WARN_ON_ONCE() when
they inevitably run on systems with copious amounts of RAM, i.e. when they
can get by KVM's "total_npages > totalram_pages()" restriction.

Note #2, KVM's usage of vmalloc()+kmalloc() instead of kvmalloc() predates
commit 7661809d493b ("mm: don't allow oversized kvmalloc() calls") by 4+
years (see commit 89c505809052 ("KVM: SVM: Add support for
KVM_SEV_LAUNCH_UPDATE_DATA command"). I.e. the open coded behavior wasn't
intended to avoid the aforementioned sanity check. The implementation
appears to be pure oversight at the time the code was written, as it showed
up in v3[1] of the early RFCs, whereas as v2[2] simply used kmalloc().

Cc: Liam Merwick <liam.merwick@oracle.com>
Link: https://lore.kernel.org/all/20170724200303.12197-17-brijesh.singh@amd.com [1]
Link: https://lore.kernel.org/all/148846786714.2349.17724971671841396908.stgit__25299.4950431914$1488470940$gmane$org@brijesh-build-machine [2]
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Link: https://patch.msgid.link/20260313003302.3136111-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>