sysctl: do not report set key in case `close_stream` fails

As we're using buffered I/O when writing kernel parameters, write errors
may get delayed until we close the `FILE` stream. As we are currently
outputting the key that is to be set disregarding the return value of
`close_stream`, we may end up in a situation where we report error and
success:

    $ sysctl kernel.printk_ratelimit=100000000000000
    sysctl: setting key "kernel.printk_ratelimit": error code 22
    kernel.printk_ratelimit = 100000000000000

Fix the issue by only outputting the updated value in case
`close_stream` does not report an error.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
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