powerpc/mm/hash: Rename get_ea_context to get_user_context

We will be adding get_kernel_context later. Update function name to indicate
this handle context allocation user space address.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>