audit: Handle embedded NUL in TTY input auditing

Data read from a TTY can contain an embedded NUL byte (e.g. after
pressing Ctrl-2, or sent to a PTY).  After the previous patch, the data
would be logged only up to the first NUL.

This patch modifies the AUDIT_TTY record to always use the hexadecimal
format, which does not terminate at the first NUL byte.  The vast
majority of recorded TTY input data will contain either ' ' or '\n', so
the hexadecimal format would have been used anyway.

Signed-off-by: Miloslav Trmac <mitr@redhat.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>