api.c: add ret value fix in cgroup_change_cgroup_flags()

api.c: add ret value fix in cgroup_change_cgroup_flags()

Fix the unused ret value warning, reported by Coverity:

CID 258304 (#1 of 1): Unused value (UNUSED_VALUE)returned_value:
Assigning value from cgroup_create_template_group(newdest, tmp, flags)
to ret here, but that stored value is overwritten before it can be used.

It turns out that cgroup_change_cgroup_flags(), calls the function
cgroup_create_template_group() to a cgroup based on a template, which
may fail and we don't check the return value for the error.  This fix
adds the check and bails out on error.

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