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.

Change-Id: I46ffb4f6aabebfa7bd66a502b6d890998b0da08c
Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>