linux-user: add sched_getattr support

linux-user: add sched_getattr support

These syscalls are not exposed by glibc. The struct type need to be
redefined as it can't be included directly before
https://lkml.org/lkml/2020/5/28/810 .

sched_attr type can grow in future kernel versions. When client sends
values that QEMU does not understand it will return E2BIG with same
semantics as old kernel would so client can retry with smaller inputs.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Message-Id: <20220105041819.24160-2-tonistiigi@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>