kfence: add note to documentation about skipping covered allocations

mainline inclusion
from mainline-v5.16-rc1
commit 5cc906b4
category: feature
bugzilla: 185780 https://gitee.com/openeuler/kernel/issues/I4EUY7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5cc906b4b4a510b274113ddb3f88d60644553f79

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Add a note briefly mentioning the new policy about "skipping currently
covered allocations if pool close to full." Since this has a notable
impact on KFENCE's bug-detection ability on systems with large uptimes,
it is worth pointing out the feature.

Link: https://lkml.kernel.org/r/20210923104803.2620285-5-elver@google.com


Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Alexander Potapenko <glider@google.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Taras Madan <tarasmadan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peng Liu <liupeng256@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>

Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>