Down the MPT adapter syscall semaphore.
int mptctl_syscall_down (
|MPT_ADAPTER * ioc, |
| ||int nonblock|
Pointer to MPT adapter
boolean, non-zero if O_NONBLOCK is set
All of the ioctl commands can potentially sleep, which is illegal
with a spinlock held, thus we perform mutual exclusion here.
Returns negative errno on error, or zero for success.