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>