l2tp: don't mask errors in pppol2tp_setsockopt()

l2tp: don't mask errors in pppol2tp_setsockopt()

commit 364700cf8fd54f54ad08313464105a414e3bccb7 upstream.

pppol2tp_setsockopt() unconditionally overwrites the error value
returned by pppol2tp_tunnel_setsockopt() or
pppol2tp_session_setsockopt(), thus hiding errors from userspace.

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>