UPSTREAM: ppp: Move PFC decompression to PPP generic layer

Extract "Protocol" field decompression code from transport protocols to
PPP generic layer, where it actually belongs. As a consequence, this
patch fixes incorrect place of PFC decompression in L2TP driver (when
it's not PPPOX_BOUND) and also enables this decompression for other
protocols, like PPPoE.

Protocol field decompression also happens in PPP Multilink Protocol
code and in PPP compression protocols implementations (bsd, deflate,
mppe). It looks like there is no easy way to get rid of that, so it was
decided to leave it as is, but provide those cases with appropriate
comments instead.

Changes in v2:
  - Fix the order of checking skb data room and proto decompression
  - Remove "inline" keyword from ppp_decompress_proto()
  - Don't split line before function name
  - Prefix ppp_decompress_proto() function with "__"
  - Add ppp_decompress_proto() function with skb data room checks
  - Add description for introduced functions
  - Fix comments (as per review on mailing list)

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

(cherry picked from commit 7fb1b8ca8fa1ee34ffc328f17f78da68c7cc04e6)
Bug: 116424816
Change-Id: Iece88d4104e9a7acc9bfe098a20ba4efcedecad6
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>