drivers/net/wan/hdlc_fr: Move the skb_headroom check out of fr_hard_header

Move the skb_headroom check out of fr_hard_header and into pvc_xmit.
This has two benefits:

1. Originally we only do this check for skbs sent by users on Ethernet-
emulating PVC devices. After the change we do this check for skbs sent on
normal PVC devices, too.
(Also add a comment to make it clear that this is only a protection
against upper layers that don't take dev->needed_headroom into account.
Such upper layers should be rare and I believe they should be fixed.)

2. After the change we can simplify the parameter list of fr_hard_header.
We no longer need to use a pointer to pointers (skb_p) because we no
longer need to replace the skb inside fr_hard_header.

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>