arm64: kfence: scale sample_interval to support early init for kfence.

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8Q3P9



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KFENCE pool requires linear map to be mapped at page granularity,
this must done in very early time in arm64. To support the late
initialisation of kfence for arm64, all mappings are turned into
page-level mappings, which incurs memory consumption.To save
memory of page table, arm64 can only map the pages in KFENCE pool
itself at page granularity. Thus, the kfence pool could not
allocated by buddy system.

For the flexibility of KFENCE, by setting "kfence.sample_interval"
to -1, the kfence pool memory will be allocated from the early
memory, kfence disabled by default, after system startup(re-enabling),
you can set "kfence.sample_interval" to a non-zero value to enable
kfence, and set "kfence.sample_interval" to 0 or -1 to turn
kfence off. Note that disabling kfence will not free the memory
associated with kfence.

Note: Whether the config KFENCE_MUST_EARLY_INIT is enabled or
not, kfence.sample_interval being set to -1 has the same function
as being 0 for non-ARM64 architectures.

Signed-off-by: Ze Zuo <zuoze1@huawei.com>