kmod/core: fix ftrace unregister order

Currently, when removing a patch module, the ftrace buffer gets flooded
with traces.  This happens because we're clearing the ftrace ops filter
before unregistering the ops, which creates a small window where all
functions are being traced.

We should be doing the unregistering in the reverse order in which we
registered, meaning ops should be unregistered and _then_ the filter
should be cleared.
Merge pull request #284 from jpoimboe/ftrace-unregister-order

kmod/core: fix ftrace unregister order