l2tp: cleanup comments

Modify some l2tp comments to better adhere to kernel coding style, as
reported by checkpatch.pl.

Add descriptive comments for the l2tp per-net spinlocks to document
their use.

Fix an incorrect comment in l2tp_recv_common:

RFC2661 section 5.4 states that:

"The LNS controls enabling and disabling of sequence numbers by sending a
data message with or without sequence numbers present at any time during
the life of a session."

l2tp handles this correctly in l2tp_recv_common, but the comment around
the code was incorrect and confusing.  Fix up the comment accordingly.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>