libcgroup: check for return code of fprintf()

libcgroup: check for return code of fprintf()

From: Vivek Goyal <vgoyal@redhat.com>

o Adding a task to a particular cgroup currently does fprintf() and never
  check the return code. Modify it to check for the return code in case
  fprintf() and fflush() did not succeed.

o I ran into the issues when i can't add migration thread to
  /container/admingroup/tasks and it will fail silently.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
git-svn-id: https://libcg.svn.sourceforge.net/svnroot/libcg/trunk@133 4f4bb910-9a46-0410-90c8-c897d4f1cd53