core: introduce CGroupIOLimitType enums

Currently, there are two cgroup IO limits, bandwidth max for read and write,
and they are hard-coded in various places.  This is fine for two limits but IO
is expected to grow more limits - low, high and max limits for bandwidth and
IOPS - and hard-coding each limit won't make sense.

This patch replaces hard-coded limits with an array indexed by
CGroupIOLimitType and accompanying string and default value tables so that new
limits can be added trivially.

src/basic/cgroup-util.c

@@ -2269,6 +2269,18 @@ int cg_weight_parse(const char *s, uint64_t *ret) {
         return 0;
 }
 
+const uint64_t cgroup_io_limit_defaults[_CGROUP_IO_LIMIT_TYPE_MAX] = {
+        [CGROUP_IO_RBPS_MAX]    = CGROUP_LIMIT_MAX,
+        [CGROUP_IO_WBPS_MAX]    = CGROUP_LIMIT_MAX,
+};
+
+static const char* const cgroup_io_limit_type_table[_CGROUP_IO_LIMIT_TYPE_MAX] = {
+        [CGROUP_IO_RBPS_MAX]    = "IOReadBandwidthMax",
+        [CGROUP_IO_WBPS_MAX]    = "IOWriteBandwidthMax",
+};
+
+DEFINE_STRING_TABLE_LOOKUP(cgroup_io_limit_type, CGroupIOLimitType);
+
 int cg_cpu_shares_parse(const char *s, uint64_t *ret) {
         uint64_t u;
         int r;