sysconfig: disable kfence in kdump kernel

In the current fedora and RHEL, below config items related to kfence
feature are set by default:

===
CONFIG_HAVE_ARCH_KFENCE=y
CONFIG_KFENCE=y
CONFIG_KFENCE_SAMPLE_INTERVAL=100
CONFIG_KFENCE_NUM_OBJECTS=255
CONFIG_KFENCE_STRESS_TEST_FAULTS=0
CONFIG_KFENCE_KUNIT_TEST=m
===

With them set, on x86_64, it will cost 2M extra memory used for kfence when
page size if 4K; while on arm64 with 64K page size, it will cost 32M extra
memory. This doesn't take memory cost of initializing and running kfence
itself into account, here only saying the kfence objects and guarded
pages. However, it doesn't make any sense to have kfence in a kdump
kernel. Hence, disable kfence in kdump kernel to save crashkernel
memory.

Author: Baoquan He

gen-kdump-sysconfig.sh

@@ -29,7 +29,7 @@ KDUMP_COMMANDLINE_REMOVE="hugepages hugepagesz slub_debug quiet log_buf_len swio
 
 # This variable lets us append arguments to the current kdump commandline
 # after processed by KDUMP_COMMANDLINE_REMOVE
-KDUMP_COMMANDLINE_APPEND="irqpoll maxcpus=1 reset_devices novmcoredd cma=0 hugetlb_cma=0"
+KDUMP_COMMANDLINE_APPEND="irqpoll maxcpus=1 reset_devices novmcoredd cma=0 hugetlb_cma=0 kfence.sample_interval=0"
 
 # This variable lets us append arguments to fadump (powerpc) capture kernel,
 # further to the parameters passed via the bootloader.