dma-mapping: mark dma_alloc_need_uncached as __always_inline

Without the __always_inline at least i386 configs that have
CONFIG_OPTIMIZE_INLINING set seem fail to inline
dma_alloc_need_uncached, leading to a linker error because of
undefined symbols.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested