inpcb: fully retire inp_ppcb pointer

Before a protocol specific control block started to embed inpcb in self
(see 0aa120d52f3c, e68b3792440c, 483fe96511ec) this pointer used to point
at it.

Retain kf_sock_inpcb field in the struct kinfo_file in <sys/user.h>. The
exp-run detected a minimal use of the field in ports:

• sysutils/lsof - patched upstream
• net-mgmt/netdata - patch accepted upstream
• emulators/qemu-user-static - upstream master branch seems not using the field anymore

We can keep the field around for some time, but eventually it may be
reused for something else.

PR: 277659 (exp-run)
Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D44491