nvmf: Narrow scope of sim lock in nvmf_sim_io

nvmf_submit_request() handles races with concurrent queue pair
destruction (or the queue pair being destroyed between
nvmf_allocate_request and nvmf_submit_request), so the lock is not
needed here. This avoids holding the lock across transport-specific
logic such as queueing mbufs for PDUs to a socket buffer, etc.

Holding the lock across nvmf_allocate_request() ensures that the queue
pair pointers in the softc are still valid as shutdown attempts will
block on the lock before destroying the queue pairs.

Sponsored by: Chelsio Communications