powerpc/uaccess: Introduce __get_user_size_goto()

We have got two places doing a goto based on the result
of __get_user_size_allowed().

Refactor that into __get_user_size_goto().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/def8a39289e02653cfb1583b3b19837de9efed3a.1615398265.git.christophe.leroy@csgroup.eu