yaffs: fix getxattr support

When passed a buffer size of 0, getxattr(2) is documented to return the
size of the attribute value.  However, yaffs returns -ERANGE instead.
This breaks SELinux usage of getxattr.  Fix yaffs to return the
attribute size in this case.

Change-Id: I410e96ad36741b8fd6aef70d0aabc22391c8d5ac
Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>
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>