yaffs: set security attribute at file creation

Add support to yaffs to set the security attribute of new inodes when
they are created. This parallels similar support in other filesystems,
and is a requirement for SELinux and other MAC systems. This support
is used by SE Android, http://selinuxproject.org/page/SEAndroid.

Change-Id: Ifad16ca6016583c9beee3b6afd2584e171297205
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>