efi/x86: Only copy the compressed kernel image in efi_relocate_kernel()

efi/x86: Only copy the compressed kernel image in efi_relocate_kernel()

The image_size argument to efi_relocate_kernel() is currently specified
as init_size, but this is unnecessarily large. The compressed kernel is
much smaller, in fact, its image only extends up to the start of _bss,
since at this point, the .bss section is still uninitialized.

Depending on compression level, this can reduce the amount of data
copied by 4-5x.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20201011142012.96493-1-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>