sockfs: getxattr: Fail with -EOPNOTSUPP for invalid attribute names

The standard return value for unsupported attribute names is
-EOPNOTSUPP, as opposed to undefined but supported attributes
(-ENODATA).

Also, fail for attribute names like "system.sockprotonameXXX" and
simplify the code a bit.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>