trivial: alloc: don't use 'top' outside spinlock

This is a fix just due to being too much of a type-A person.
I noticed the issue while reading over the function, and
decided to fix it, even though it's unlikely to be a problem
ever because top is read-mostly (like written once, then only
read, type of mostly).

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <1446769483-21586-4-git-send-email-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>