[PATCH kexec-tools v3] arm-zImage: prepare for more malloc requirements of zstd

[Tom Fitzhenry]

I tested this in QEMU with kexec-tools at 43c5cba85ad2 plus this patch:

1. Against unmodified Linux 8d3ae59288f1 (gzip): kexec reads the
    64 KiB malloc size from the header and boots the second kernel via
    kexec -e without regression:

   # /bin/kexec-patched --debug -l /boot/zImage 
--initrd=/boot/tiny-initrd.cpio.gz --append="console=ttyAMA0 kexec_booted=1"
   [...]
   zImage size 0x1a3500, file size 0x1a3500
     offset 0x00007770 tag 0x5a534c4b size 24
   zImage requires 0x001b4500 bytes
   [...]

   $ printf "0x%x\n" $((0x001b4500 - 0x001a3500))
   0x11000  # 0x1000 (4 KiB stack) + 0x10000 (64 KiB malloc)

2. Against Linux 8d3ae59288f1 with the v2 ARM zstd series[1] applied:
    kexec reads the increased 128 KiB malloc size from the
    header and boots the second kernel via kexec -e:

   # /bin/kexec-patched --debug -l /boot/zImage 
--initrd=/boot/tiny-initrd.cpio.gz --append="console=ttyAMA0 kexec_booted=1"
   [...]
   zImage size 0x182548, file size 0x182548
     offset 0x00017370 tag 0x5a534c4b size 24
   zImage requires 0x001a3548 bytes
   [...]

   $ printf "0x%x\n" $((0x001a3548 - 0x00182548))
   0x21000  # 0x1000 (4 KiB stack) + 0x20000 (128 KiB malloc)

Thank you addressing my comments.

Reviewed-by: Tom Fitzhenry <tom at tom-fitzhenry.me.uk>
Tested-by: Tom Fitzhenry <tom at tom-fitzhenry.me.uk>

[1] 
https://lore.kernel.org/lkml/20260828-zstd2-v2-0-4d9ce7150877@kemnade.info/