api.c: add precision to fscanf(), in cgroup_get_current_controller_path()

api.c: add precision to fscanf(), in cgroup_get_current_controller_path()

Fix calling risky function warning, reported by Coverity tool:

CID 258301 (#1 of 1): Calling risky function
(DC.STREAM_BUFFER)dont_call: fscanf assumes an arbitrarily long string,
so callers must use correct precision specifiers or never use fscanf.

As per secure coding standard, using '%s' in the fscanf() is not
recommend, hence fix it by using the precision of macro
FILENAME_MAX borrowed from Linux Kernel for the maximum
allowed controller/subsys_name length.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
TJH: Small fix in the commit comment.