unix: fix small race in fs.c
unix: fix small race in fs.c

Don't return req->result after posting the work request. It's possible (if
unlikely) for a worker thread to process the request before the main thread
reaches the return statement.

unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso