dma-mapping: don't allow DMA map to be marked exec

DMA mapping permissions were being derived from pgprot_kernel directly
without using PAGE_KERNEL.  This causes them to be marked with executable
permission, which is not what we want.  Fix this.
Credit goes to CAF.

Change-Id: Ib40f59f3c569f82409943cf8f9a86a9869d922cc