Handle copyin failures.

Skip the log entry as there is nothing good to write out. Don't fail
the syscall though since it already succeeded. There's no reason
filemon's tracing failure should fail the already-succeeded syscall.

Record the error for later to return from close(2) on the filemon devfs
file descriptor.

Discussed with: markj, sjg, kib (briefly with kib)
Reported by: mjg
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division