lsm_audit: avoid overloading the "key" audit field

The "key" field is used to associate records with the rule that
triggered them, os it's not a good idea to overload it with an
additional IPC key semantic. Moreover, as the classic "key" field is a
text field, while the IPC key is numeric, AVC records containing the IPC
key info actually confuse audit userspace, which tries to interpret the
number as a hex-encoded string, thus showing garbage for example in the
ausearch "interpret" output mode.

Hence, change it to "ipc_key" to fix both issues and also make the
meaning of this field more clear.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>