xtensa: fix type conversion in __get_user_size

xtensa: fix type conversion in __get_user_size

8-byte access in __get_user_size converts pointer to temporary variable
to the type of original user pointer and then dereferences it, resulting
in the following sparse warning:

  sparse: warning: dereference of noderef expression

Instead dereference the original user pointer under the __typeof__ and
add indirection outside.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>