Simplify some logic by merging an if test with a subsequent switch.

Specifically, in aio_queue_file() the code was doing this:

if (opcode == LIO_SYNC) {
    ...
}

switch (opcode) {
...
case LIO_SYNC:
    ...
}

This moves the body of the if statement into the LIO_SYNC case of the
switch statement.

MFC after: 2 weeks
Sponsored by: Chelsio Communications