x86/boot/compressed/64: Use 32-bit (zero-extended) MOV for z_output_len

z_output_len is the size of the decompressed payload (i.e. vmlinux +
vmlinux.relocs) and is generated as an unsigned 32-bit quantity by
mkpiggy.c.

The current

  movq $z_output_len, %r9

instruction generates a sign-extended move to %r9. Using

  movl $z_output_len, %r9d

will instead zero-extend into %r9, which is appropriate for an unsigned
32-bit quantity. This is also what is already done for z_input_len, the
size of the compressed payload.

[ bp:

  Also, z_output_len cannot be a 64-bit quantity because it participates
  in:

  init_size:              .long INIT_SIZE         # kernel initialization size

  through INIT_SIZE which is a 32-bit quantity determined by the .long
  directive (vs .quad for 64-bit). Furthermore, if it really must be a
  64-bit quantity, then the insn must be MOVABS which can accommodate a
  64-bit immediate and which the toolchain does not generate automatically.
]

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200211173333.1722739-1-nivedita@alum.mit.edu