net/netrom: Fix socket locking

upstream commit: cc29c70dd581f85ee7a3e7980fb031f90b90a2ab

Patch "af_rose/x25: Sanity check the maximum user frame size"
(commit 83e0bbcbe2145f160fbaa109b0439dae7f4a38a9) 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>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>