os: un-duplicate code to close client on write failure

There are three copies of the same short sequence of operations to
close down a client when a write error occurs. Create a new function,
AbortClient, which performs these operations and then call it from the
three places.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>