dahdi: Protect echocan creation/destruction with mutex.

This closes a reference and memory leak when multiple CPUs are enabling echocan
on a single channel in parallel.

The essential problem is that the call to try_module_get() is not serialized.
Two separate threads can come into ioctl_echocan() on the same channel, they
coordinate via the dahdi_chan.lock to release any current echocan, but then both
create a new echocan state, bump the reference on the module, and the last one
through will actually attach the new state to the channel. The earlier reference
/ memory is leaked.

I tried to conceive of a way to fix this leak without adding a new lock, but the
choices where calling throught the function pointers with dahdi_chan.lock.
Otherwise I needed to change the semantics of echocan_create /free which would
ripple through the hardware echocan modules.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>

include/dahdi/kernel.h

@@ -425,6 +425,7 @@ struct dahdi_chan {
 	int lastnumbufs;
 #endif
 	spinlock_t lock;
+	struct mutex mutex;
 	char name[40];
 	/* Specified by DAHDI */
 	/*! \brief DAHDI channel number */