Avoid bind lock recursion.

When dlclose(3) unloads an object with filtees, it recursively calls
dlclose(3) on each filtee in free_needed_filtees(). Introduce
dlclose_locked() helper, called from free_needed_filtees() instead of
dlclose(), and pass the bind lockstate down to avoid recursing.

Reported and tested by: jhibbits
Sponsored by: The FreeBSD Foundation
MFC after: 1 week