The VFS calls backends from goroutines of its own. A panic on any of these
cannot be recovered. So a backend panicking on a single file killed the whole
process, taking down a mount or every user of a serve command, even for servers
such as serve http whose library recovers panics raised on its own request
goroutines.
Recover panics at those goroutines and log them with a stack trace. Where the
surrounding code already handles a failure, recover around the backend call
itself rather than the whole goroutine, so a panicking upload is retried like
any other failed upload and a panicking download is reported to the waiters,
instead of abandoning the work part way through and leaving the bookkeeping
inconsistent.
Addresses GHSA-6jcg-q3wp-x2f4
