powerpc: Fix build break due to paca mm_context_t changes

Commit 2fc251a8dd ("powerpc: Copy only required pieces of the
mm_context_t to the paca") broke the build for CONFIG_PPC_STD_MMU_64=y
and CONFIG_PPC_MM_SLICES=n.

That only happens for a kernel built with 4K pages and HUGETLB disabled,
which is why we missed it.

Fix it by adding a mm_ctx_user_psize member to the paca and populating
it in the appropriate places.

Fixes: 2fc251a8dd ("powerpc: Copy only required pieces of the mm_context_t to the paca")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hash_utils_64.c

@@ -866,7 +866,7 @@ static unsigned int get_paca_psize(unsigned long addr)
 #else
 unsigned int get_paca_psize(unsigned long addr)
 {
-	return get_paca()->context.user_psize;
+	return get_paca()->mm_ctx_user_psize;
 }
 #endif