lib/vmalloc: fix pages count local variable to be size_t

Since size is of type size_t, size >> PAGE_SHIFT might still be too big
for a normal unsigned int.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20200706164324.81123-2-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>