Eliminate unnecessary use of labels in error unwinding cases; result is
the same number or fewer lines of code.

Don't cast using caddr_t.

Remember to unlock the natm lock in some error cases where it was leaked
previously.

Annotate two cases where we'd like to hold the natm subsystem lock over
ioctls into the device driver.

Hold the natm lock longer in natm_usr_connect() so we can copy the npcb
fields while holding the mutex.

MFC after: 3 months