SUNRPC: Don't return error values in sysfs read of closed files

SUNRPC: Don't return error values in sysfs read of closed files

Instead of returning an error value, which ends up being the return
value for the read() system call, it is more elegant to simply return
the error as a string value.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>