src/api.c: fix TOCTOU in cgroup_get_cgroup()

src/api.c: fix TOCTOU in cgroup_get_cgroup()

Fix a TOCTOU issue, reported by Coverity tool:

CID 258268 (#1 of 1): Time of check time of use (TOCTOU)28.
fs_check_call: Calling function access to perform check on path.

Coverity gets confused when a char array is re-used for constructing
different paths and complains about TOCTOU for unrelated paths, fix it
by using different char arrays for different path, as a side effect it
also improves the readability of the code.

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