pybind/cephfs: Fix setxattr function. Pass value as character pointer

This passes the value of the xattr correctly and makes it work.

src/pybind/cephfs.py

@@ -333,7 +333,7 @@ def setxattr(self, path, name, value, flags):
                     self.cluster,
                     c_char_p(path),
                     c_char_p(name),
-                    c_void_p(value),
+                    c_char_p(value),
                     c_size_t(len(value)),
                     c_int(flags))
         if ret < 0: