common: make helpers for ttyprintk usage

common: make helpers for ttyprintk usage

A handful of tests write things to /dev/ttyprintk to make it easier to
pinpoint where in a test something went wrong.  This isn't entirely
robust, however, because ttyprintk is an optional feature.  In the grand
tradition of kernel design there's also a /dev/kmsg that does nearly the
same thing, is also optional, and there's no documentation spelling out
when one is supposed to use one or the other.

So.

Create a pair of helpers to append messages to the kernel log.  One
simply writes its arguments to the kernel log, and the other writes
stdin to the kernel log, stdout, and any other files specified as
arguments.

Underneath the covers, both functions will send the message to
/dev/ttyprintk if available.  If it isn't but /dev/kmsg is, they'll
send the messages there, prepending a "[U]" to emulate the only
discernable difference between ttyprintk and kmsg.

If neither are available, then either /dev or the kernel aren't allowing
us to write to the kernel log, and the messages are not logged.  The
second helper will still write the messages to stdout.

If this seems like overengineered nonsense, then yes it is.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>