api: leave room for NUL in cgroup_get_procs()

api: leave room for NUL in cgroup_get_procs()

Commit 4a851a569689 ("api: Fix unsafe call to strncat in
cgroup_get_procs() and cgroup_get_threads()") tightened several
strncat() callers, but the code building the procs/threads paths
still used the raw remaining buffer size. When the buffer is
already full, strncat() would copy path_sz bytes and overwrite
the terminator. Subtract one from the remaining length before
appending "tasks" or the controller-specific filename so there
is always space for the trailing NUL.

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