add user_struct element unix_inflight pre-req for "pipe: limit the per-user amount...

add user_struct element unix_inflight pre-req for "pipe: limit the per-user amount of pages allocated in pipes"

The commit: pipe: limit the per-user amount of pages allocated in pipes
caused a merge conflict with uek/uek-4.1 due to:
c057264c4ba7ddf9d775c013fcb1cecb910bf2b9
fix kABI breakage from "unix: properly account for FDs passed over unix sock

Add include/linux/sched.h user_struct change from c057264c so that the
code context matches uek/uek-4.1

Orabug: 22901731
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>