dahdi: Add module parameter to limit number of pseudo channels.

Since there isn't a fixed array to hold all the channels, and also limit
the total number of channels that can be created, we'll add a module
parameter to allow the system administrator to specify the maximum
number of pseudo channels.  This is to prevent a potentially
non-privledged process from consuming too much CPU (since all pseudos
are checked each "tick" for conferencing) and kernel memory.

By default the number of pseudo channels is limited to 512.  Change the
default limit with:

]# modprobe dahdi max_pseudo_channels=<new limit>

or at runtime with:

]# echo <new limit> > /sys/module/dahdi/parameters/max_psuedo_channels

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Kinsey Moore <kmoore@digium.com>

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@9610 a0bf4364-ded3-4de4-8d8a-66a801d63aff

README

@@ -437,6 +437,11 @@ deftaps (dahdi)::
   different default size. So normally setting this doesn't change
   anything.
 
+max_pseudo_channels (dahdi)::
+  The maximum number of pseudo channels that dahdi will allow userspace to
+  create.  Pseudo channels are used when conferencing channels together.
+  The default is 512.
+
 To get a list of parameters supported by a module, use 
 
   modinfo module_name