x86: unify kmap_atomic_pfn() and iomap_atomic_prot_pfn()

kmap_atomic_pfn() and iomap_atomic_prot_pfn() are almost same
except pgprot. This patch removes the code duplication for these
two functions.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
LKML-Reference: <20090311143317.GA22244@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>