ipmi: Add a limit on the number of users that may use IPMI

Each user uses memory, we need limits to avoid a rogue program from
running the system out of memory.

Based on work by Chen Guanqiao <chen.chenchacha@foxmail.com>

Cc: Chen Guanqiao <chen.chenchacha@foxmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>