sched_rr_get_interval01 from ubuntu_ltp_syscalls failed on 4.4

== SRU Justification ==
A new regression test case "sched_rr_get_interval01" for commit 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds"). Has been added to the LTP syscalls test suite.

On Xenial 4.4.0-186.216, this test is failing with:

 startup='Wed Jul 8 10:36:58 2020'
 tst_test.c:1247: INFO: Timeout per run is 0h 05m 00s
 sched_rr_get_interval01.c:43: INFO: Testing variant: vDSO or syscall with libc spec
 sched_rr_get_interval01.c:60: PASS: sched_rr_get_interval() passed
 sched_rr_get_interval01.c:68: PASS: Time quantum 0s 100000000ns
 sched_rr_get_interval01.c:75: FAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 25
 tst_test.c:1247: INFO: Timeout per run is 0h 05m 00s
 sched_rr_get_interval01.c:43: INFO: Testing variant: syscall with old kernel spec
 sched_rr_get_interval01.c:60: PASS: sched_rr_get_interval() passed
 sched_rr_get_interval01.c:68: PASS: Time quantum 0s 100000000ns
 sched_rr_get_interval01.c:75: FAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 25

 HINT: You _MAY_ be missing kernel fixes, see:

 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=975e155ed873

 Summary:
 passed 4
 failed 2
 skipped 0
 warnings 0
 tag=sched_rr_get_interval01 stime=1594204618 dur=0 exit=exited stat=1 core=no cu=0 cs=0

== Fixes ==
975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds")

This patch can be cherry-picked in to the affected Xenial kernel. It's already been applied in newer kernels.

== Test ==
Test kernel can be found here:
https://people.canonical.com/~phlin/kernel/lp-1886890-sched_rr_get_interval01/

With the patch applied, this test will pass without any issue.

And it can be simply tested with:
# echo 100 > /proc/sys/kernel/sched_rr_timeslice_ms
# cat /proc/sys/kernel/sched_rr_timeslice_ms
100
(was 25 before the patch)