log: add log level string prefix to logging functions

log: add log level string prefix to logging functions

Usage of cgroup_{err, warn, info} functions, expects the log level to be
prefixed to the message like: cgroup_warn("Warning: ...").  This patch
removes this need to prefix and automatically adds the desired prefix to
the logger function, so the usage becomes cgroup_warn("...").  This
helps in readability and avoiding typos in setting the right log level
string to the message.

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