xattr: Remove unnecessary NULL attribute name check

When NULL is passed to one of the xattr system calls as the attribute
name, copying that name from user space already fails with -EFAULT;
xattr_resolve_name is never called with a NULL attribute name.

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