coda: fix 'kernel memory exposure attempt' in fsync

When an application called fsync on a file in Coda a small request with
just the file identifier was allocated, but the declared length was set
to the size of union of all possible upcall requests.

This bug has been around for a very long time and is now caught by the
extra checking in usercopy that was introduced in Linux-4.8.

The exposure happens when the Coda cache manager process reads the fsync
upcall request at which point it is killed. As a result there is nobody
servicing any further upcalls, trapping any processes that try to access
the mounted Coda filesystem.

Cc: stable@vger.kernel.org
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>