powerpc/32: Automatically adapt TASK_SIZE based on constraints

At the time being, TASK_SIZE can be customized by the user via Kconfig
but it is not possible to check all constraints in Kconfig. Impossible
setups are detected at compile time with BUILD_BUG() but that leads
to build failure when setting crazy values. It is not a problem on its
own because the user will usually either use the default value or set
a well thought value. However build robots generate crazy random
configs that lead to build failures, and build robots see it as a
regression every time a patch adds such a constraint.

So instead of failing the build when the custom TASK_SIZE is too
big, just adjust it to the maximum possible value matching the setup.

Several architectures already calculate TASK_SIZE based on other
parameters and options.

In order to do so, move MODULES_VADDR calculation into task_size_32.h
and ensure that:
- On book3s/32, userspace and module area have their own segments (256M)
- On 8xx, userspace has its own full PGDIR entries (4M)

Then TASK_SIZE is guaranteed to be correct so remove related
BUILD_BUG()s.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/6a2575420770d075cd090b5a316730a2ffafdee4.1766574657.git.chleroy@kernel.org

arch/powerpc/include/asm/book3s/32/pgtable.h

@@ -195,10 +195,6 @@ void unmap_kernel_page(unsigned long va);
 #define VMALLOC_END	ioremap_bot
 #endif
 
-#define MODULES_END	ALIGN_DOWN(PAGE_OFFSET, SZ_256M)
-#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
-#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
-
 #ifndef __ASSEMBLER__
 #include <linux/sched.h>
 #include <linux/threads.h>