[PATCH] minor audit updates

Just a few minor proposed updates.  Only the last one will
actually affect behavior.  The rest are just misleading
code.

Several AUDIT_SET functions return 'old' value, but only
return value <0 is checked for.  So just return 0.

propagate audit_set_rate_limit and audit_set_backlog_limit
error values

In audit_buffer_free, the audit_freelist_count was being
incremented even when we discard the return buffer, so
audit_freelist_count can end up wrong.  This could cause
the actual freelist to shrink over time, eventually
threatening to degrate audit performance.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>