passthrough: Reset config on FUSE_DESTROY
PassthroughFs::{writeback, announce_submounts}
are set (as the
circumstances require) in init()
. Therefore, we also reset them
in destroy()
. Without this commit, those variables would always
be set after they're set once. This might be undesired when the
caller calls destroy()
and then init()
with deviating
FsOptions
.