api.c: check the bytes read in cgroup_register_unchanged_process()

api.c: check the bytes read in cgroup_register_unchanged_process()

Fix ignoring the number of bytes read, warning reported by Coverity
tool:

CID 258288 (#1 of 1): Ignoring number of bytes read (CHECKED_RETURN).
check_return: read(int, void *, size_t) returns the number of bytes
read, but it is ignored.

In cgroup_register_unchanged_process(), the number of byte read/written
using read()/write() are ignored but coverity it warns about the read()
only, let's fix it.

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