closures: closure_sync_timeout()

closures: closure_sync_timeout()

Add a new variant of closure_sync_timeout() that takes a timeout.

Note that when this returns -ETIME the closure will still be waiting on
something, i.e. it's not safe to return if you've got a stack allocated
closure.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>