net/tls: use sg_next() to walk sg entries

Partially sent record cleanup path increments an SG entry
directly instead of using sg_next(). This should not be a
problem today, as encrypted messages should be always
allocated as arrays. But given this is a cleanup path it's
easy to miss was this ever to change. Use sg_next(), and
simplify the code.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>