It has been reported some configuration where the kernel doesn't
boot with KASAN enabled.

This is due to wrong BAT allocation for the KASAN area:

	---[ Data Block Address Translation ]---
	0: 0xc0000000-0xcfffffff 0x00000000       256M Kernel rw      m
	1: 0xd0000000-0xdfffffff 0x10000000       256M Kernel rw      m
	2: 0xe0000000-0xefffffff 0x20000000       256M Kernel rw      m
	3: 0xf8000000-0xf9ffffff 0x2a000000        32M Kernel rw      m
	4: 0xfa000000-0xfdffffff 0x2c000000        64M Kernel rw      m

A BAT must have both virtual and physical addresses alignment matching
the size of the BAT. This is not the case for BAT 4 above.

Fix kasan_init_region() by using block_size() function that is in
book3s32/mmu.c. To be able to reuse it here, make it non static and
change its name to bat_block_size() in order to avoid name conflict
with block_size() defined in <linux/blkdev.h>

Also reuse find_free_bat() to avoid an error message from setbat()
when no BAT is available.

And allocate memory outside of linear memory mapping to avoid
wasting that precious space.

With this change we get correct alignment for BATs and KASAN shadow
memory is allocated outside the linear memory space.

	---[ Data Block Address Translation ]---
	0: 0xc0000000-0xcfffffff 0x00000000       256M Kernel rw
	1: 0xd0000000-0xdfffffff 0x10000000       256M Kernel rw
	2: 0xe0000000-0xefffffff 0x20000000       256M Kernel rw
	3: 0xf8000000-0xfbffffff 0x7c000000        64M Kernel rw
	4: 0xfc000000-0xfdffffff 0x7a000000        32M Kernel rw

Fixes: 7974c473 ("powerpc/32s: Implement dedicated kasan_init_region()")
Cc: stable@vger.kernel.org
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7a50ef902494d1325227d47d33dada01e52e5518.1641818726.git.christophe.leroy@csgroup.eu