virt: guest_memfd: fix reference leak on hwpoisoned page

If kvm_gmem_get_pfn() detects an hwpoisoned page, it returns -EHWPOISON
but it does not put back the reference that kvm_gmem_get_folio() had
grabbed.  Add the forgotten folio_put().

Fixes: a7800aa8 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Cc: stable@vger.kernel.org
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/guest_memfd.c

@@ -510,8 +510,10 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	}
 
 	if (folio_test_hwpoison(folio)) {
+		folio_unlock(folio);
+		folio_put(folio);
 		r = -EHWPOISON;
-		goto out_unlock;
+		goto out_fput;
 	}
 
 	page = folio_file_page(folio, index);
@@ -522,7 +524,6 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 
 	r = 0;
 
-out_unlock:
 	folio_unlock(folio);
 out_fput:
 	fput(file);