api.c: fix segfault in cgroup_populate_mount_points()

api.c: fix segfault in cgroup_populate_mount_points()

In cgroup_populate_mount_points(),  cgroup v1/v2 mount points get read
from /proc/mounts and populated into cg_mount_table[].  The size of
cg_mount_table[] is set to CG_CONTROLLER_MAX, that's defined as 100 and
if the system has more than CG_CONTROLLER_MAX, unique mount points, it
will segfault while processing them.  Fix this by checking, the mount
point count after processing every mount entry and bailout in case it
reaches CG_CONTROLLER_MAX.

The issue can be reproduced using, following simple bash commands on
cgroup v1:
1. sudo for i in $(seq 0 100); do sudo mkdir /name$i; done
2. sudo for i in $(seq 0 86); do sudo mount -t cgroup -o none,name=named$i
none /name$i; done
3. sudo cgget -g <controller>:<existing cgroup name>

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>