[AVR32] Simplify pte_alloc_one{,_kernel}

There's really no need to retry an allocation with __GFP_REPEAT set.
Also, use get_zeroed_page() and __GFP_ZERO to eliminate the extra call
to clear_page() afterwards.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>