cgroup: fix memory cgroup limit regression on kernel 3.10 (#3673)

cgroup: fix memory cgroup limit regression on kernel 3.10 (#3673)

Commit da4d897e ("core: add cgroup memory controller support on the unified
hierarchy (#3315)") changed the code in src/core/cgroup.c to always write
the real numeric value from the cgroup parameters to the
"memory.limit_in_bytes" attribute file.

For parameters set to CGROUP_LIMIT_MAX, this results in the string
"18446744073709551615" being written into that file, which is UINT64_MAX.
Before that commit, CGROUP_LIMIT_MAX was special-cased to the string "-1".

This causes a regression on CentOS 7, which is based on kernel 3.10, as the
value is interpreted as *signed* 64 bit, and clamped to 0:

[root@n54 ~]# echo 18446744073709551615 >/sys/fs/cgroup/memory/user.slice/memory.limit_in_bytes
[root@n54 ~]# cat /sys/fs/cgroup/memory/user.slice/memory.limit_in_bytes
0

[root@n54 ~]# echo -1 >/sys/fs/cgroup/memory/user.slice/memory.limit_in_bytes
[root@n54 ~]# cat /sys/fs/cgroup/memory/user.slice/memory.limit_in_bytes
9223372036854775807

Hence, all units that are subject to the limits enforced by the memory
controller will crash immediately, even though they have no actual limit
set. This happens to for the user.slice, for instance:

[  453.577153] Hardware name: SeaMicro SM15000-64-CC-AA-1Ox1/AMD Server CRB, BIOS Estoc.3.72.19.0018 08/19/2014
[  453.587024]  ffff880810c56780 00000000aae9501f ffff880813d7fcd0 ffffffff816360fc
[  453.594544]  ffff880813d7fd60 ffffffff8163109c ffff88080ffc5000 ffff880813d7fd28
[  453.602120]  ffffffff00000202 fffeefff00000000 0000000000000001 ffff880810c56c03
[  453.609680] Call Trace:
[  453.612156]  [<ffffffff816360fc>] dump_stack+0x19/0x1b
[  453.617324]  [<ffffffff8163109c>] dump_header+0x8e/0x214
[  453.622671]  [<ffffffff8116d20e>] oom_kill_process+0x24e/0x3b0
[  453.628559]  [<ffffffff81088dae>] ? has_capability_noaudit+0x1e/0x30
[  453.634969]  [<ffffffff811d4155>] mem_cgroup_oom_synchronize+0x575/0x5a0
[  453.641721]  [<ffffffff811d3520>] ? mem_cgroup_charge_common+0xc0/0xc0
[  453.648299]  [<ffffffff8116da84>] pagefault_out_of_memory+0x14/0x90
[  453.654621]  [<ffffffff8162f4cc>] mm_fault_error+0x68/0x12b
[  453.660233]  [<ffffffff81642012>] __do_page_fault+0x3e2/0x450
[  453.666017]  [<ffffffff816420a3>] do_page_fault+0x23/0x80
[  453.671467]  [<ffffffff8163e308>] page_fault+0x28/0x30
[  453.676656] Task in /user.slice/user-0.slice/user@0.service killed as a result of limit of /user.slice/user-0.slice/user@0.service
[  453.688477] memory: usage 0kB, limit 0kB, failcnt 7
[  453.693391] memory+swap: usage 0kB, limit 9007199254740991kB, failcnt 0
[  453.700039] kmem: usage 0kB, limit 9007199254740991kB, failcnt 0
[  453.706076] Memory cgroup stats for /user.slice/user-0.slice/user@0.service: cache:0KB rss:0KB rss_huge:0KB mapped_file:0KB swap:0KB inactive_anon:0KB active_anon:0KB inactive_file:0KB active_file:0KB unevictable:0KB
[  453.725702] [ pid ]   uid  tgid total_vm      rss nr_ptes swapents oom_score_adj name
[  453.733614] [ 2837]     0  2837    11950      899      23        0             0 (systemd)
[  453.741919] Memory cgroup out of memory: Kill process 2837 ((systemd)) score 1 or sacrifice child
[  453.750831] Killed process 2837 ((systemd)) total-vm:47800kB, anon-rss:3188kB, file-rss:408kB

Fix this issue by special-casing the UINT64_MAX case again.