From: Trond Myklebust <trond.myklebust@fys.uio.no>

If users set the execute bit on a file, and then write to it,
remove_suid() causes a flood of SETATTR calls (one per write() syscall)
with no arguments to be sent down the wire.

The server will in any case clear the suid bit itself without any
prompting from us, so the following patch simply filters away all
SETATTR requests with empty or unsupported ia_valid fields.