net/netrom: Fix socket locking

Patch "af_rose/x25: Sanity check the maximum user frame size"
(commit 83e0bbcb) from Alan Cox got
locking wrong. If we bail out due to user frame size being too large,
we must unlock the socket beforehand.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>