ppp: fix lost fragments in ppp_mp_explode() (resubmit)

This patch fixes the corner cases where the sum of MTU of the free
channels (adjusted for fragmentation overheads) is less than the MTU
of PPP link.  There are at least 3 situations where this case might
arise:

- some of the channels are busy

- the multilink session is running in a degraded state (i.e. with less
than its full complement of active channels)

- by design, where multilink protocol is being used to artificially
increase the effective link MTU of a single link.

Without this patch, at most 1 fragment is ever sent per free channel
for a given PPP frame and any remaining part of the PPP frame that
does not fit into those fragments is silently discarded.

This patch restores the original behaviour which was broken by commit
9c705260 'ppp:ppp_mp_explode()
redesign'.  Once all 'free' channels have been given a fragment, an
additional fragment is queued to each available channel in turn, as many
times as necessary, until the entire PPP frame has been consumed.

Signed-off-by: Ben McKeegan <ben@netservers.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>