ppp_generic: Help GCC see that 'flen' is always initialized.

It's too stupid to see that we always set flen to something
before we use it in ppp_mp_explode():

drivers/net/ppp_generic.c: In function 'ppp_push':
drivers/net/ppp_generic.c:1314: warning: 'flen' may be used uninitialized in this function
drivers/net/ppp_generic.c:1314: note: 'flen' was declared here

This started warning after commit a53a8b56
("ppp: fix lost fragments in ppp_mp_explode() (resubmit)")

So just put an explicit unconditional initialization there to
hush it up.

Signed-off-by: David S. Miller <davem@davemloft.net>