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.

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: I4038621dc92af43573ec022a7cf09a0ec1bd5870
Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>