bootm: increase kernel_noload decompression headroom from 4x to 8x

bootm: increase kernel_noload decompression headroom from 4x to 8x

For a compressed kernel_noload image, bootm_load_os() allocates a buffer
of ALIGN(image_len * 4, SZ_1M). The 4x factor is at the edge of what
modern compressors (zstd, xz) achieve on real kernels, so a
well-compressed vendor kernel can fail to boot at runtime with no
intervening warning.

Bump the headroom to 8x. The buffer is still bounded by the compressed
image size, and the SZ_1M alignment keeps the overhead below 1 MiB on
small kernels.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Aristo Chen <aristo.chen@canonical.com>