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 e.g. SELinux usage of getxattr.  Fix yaffs to return the
attribute size in this case.

Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>
Change-Id: I7c54ac339c8c2dc24d21607f1b505ed4a1826c14