[warm-reboot] Use kexec_file_load instead of kexec_load when available (

#2608)

On some dev VMs, warm reboot on a VS image fails. Specifically, after
kexec is called and the new kernel starts, the new kernel tries to load
the initramfs, but fails to do so for whatever reason. There may be
messages about gzip decompression failing and that it's corrupted.

After some experimentation, it was found that when first loading the new
kernel and initramfs into memory, using the `kexec_file_load` syscall
(`-s` flag in kexec) worked fine, whereas using the default `kexec_load`
syscall resulted in a failure. It's unknown why `kexec_file_load` worked
fine when `kexec_load` didn't; there shouldn't be any difference for
non-secure boot kernels, as far as I can tell. What was seen, however,
was that when taking a KVM dump in the failure case, the memory that
stored the initramfs had differences compared to what was on disk. It's
unknown what caused these differences.

As a workaround (and as a bit of a feature enhancement), use the `-a`
flag with kexec, which tells it to use `kexec_file_load` if available,
and `kexec_load` if it's not available or otherwise fails. armhf doesn't
support `kexec_file_load`, whereas arm64 gained support for
`kexec_file_load` in the 5.19 kernel (we're currently on 5.10). `amd64`
has supported `kexec_file_load` since 3.17. This also makes it possible
to do kexec on secure boot systems, where the kernel image must be
loaded via `kexec_file_load`.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

scripts/fast-reboot

@@ -439,7 +439,7 @@ function load_aboot_secureboot_kernel() {
 
 function load_kernel() {
     # Load kernel into the memory
-    /sbin/kexec -l "$KERNEL_IMAGE" --initrd="$INITRD" --append="$BOOT_OPTIONS"
+    /sbin/kexec -a -l "$KERNEL_IMAGE" --initrd="$INITRD" --append="$BOOT_OPTIONS"
 }
 
 function unload_kernel()