powerpc/mm: Fix null-pointer dereference in pgtable_cache_add

mainline inclusion
from mainline-v6.8-rc1
commit f46c8a75263f97bda13c739ba1c90aced0d3b071
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I96GP1
CVE: CVE-2023-52607

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f46c8a75263f97bda13c739ba1c90aced0d3b071



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kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful
by checking the pointer validity.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231204023223.2447523-1-chentao@kylinos.cn


Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>