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>