initramfs: fix populate_initrd_image() section mismatch

With gcc-4.6.3:

    WARNING: vmlinux.o(.text.unlikely+0x140): Section mismatch in reference from the function populate_initrd_image() to the variable .init.ramfs.info:__initramfs_size
    The function populate_initrd_image() references
    the variable __init __initramfs_size.
    This is often because populate_initrd_image lacks a __init
    annotation or the annotation of __initramfs_size is wrong.

    WARNING: vmlinux.o(.text.unlikely+0x14c): Section mismatch in reference from the function populate_initrd_image() to the function .init.text:unpack_to_rootfs()
    The function populate_initrd_image() references
    the function __init unpack_to_rootfs().
    This is often because populate_initrd_image lacks a __init
    annotation or the annotation of unpack_to_rootfs is wrong.

    WARNING: vmlinux.o(.text.unlikely+0x198): Section mismatch in reference from the function populate_initrd_image() to the function .init.text:xwrite()
    The function ...