powerpc/uaccess: Merge __put_user_size_allowed() into __put_user_size()

__put_user_size_allowed() is only called from __put_user_size() now.

Merge them together.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b3baeaec1ee2fbdc653bb6fb27b0be5b846163ef.1612879284.git.christophe.leroy@csgroup.eu