xen: annotate functions which only call into __init at start of day

Both xen_hvm_init_shared_info and xen_build_mfn_list_list can be
called at resume time as well as at start of day but only reference
__init functions (extend_brk) at start of day. Hence annotate with
__ref.

    WARNING: arch/x86/built-in.o(.text+0x4f1): Section mismatch in reference
        from the function xen_hvm_init_shared_info() to the function
        .init.text:extend_brk()
    The function xen_hvm_init_shared_info() references
    the function __init extend_brk().
    This is often because xen_hvm_init_shared_info lacks a __init
    annotation or the annotation of extend_brk is wrong.

xen_hvm_init_shared_info calls extend_brk() iff !shared_info_page and
initialises shared_info_page with the result. This happens at start of
day only.

    WARNING: arch/x86/built-in.o(.text+0x599b): Section mismatch in reference
        from the function xen_build_mfn_list_list() to the function
        .init.text:extend_brk()
    The function xen_build_mfn_list_list() references
    the function __init extend_brk().
    This is often because xen_build_mfn_list_list lacks a __init
    annotation or the annotation of extend_brk is wrong.

(this warning occurs multiple times)

xen_build_mfn_list_list only calls extend_brk() at boot time, while
building the initial mfn list list

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>