GEOM: Switch g_io_deliver() locking from cp to pp.

Single provider may have multiple consumers, and locking one of consumers
is not sufficient to protect the provider. Though the only part of the
provider this locking protects now is its statistics.

Reported by: Arka Sharma <arka.sw1988@gmail.com>
MFC after: 2 weeks

(cherry picked from commit 06bd74e1e39cb33b18b19a748a80b5e1d70fda17)