[SPARC64]: Fix section mismatch from kernel_map_range

Fix following warnings:
WARNING: vmlinux.o(.text+0x4f980): Section mismatch in reference from the function kernel_map_range() to the function .init.text:__alloc_bootmem()
WARNING: vmlinux.o(.text+0x4f9cc): Section mismatch in reference from the function kernel_map_range() to the function .init.text:__alloc_bootmem()

alloc_bootmem() is only used during early init and for any subsequent
call to kernel_map_range() the program logic avoid the call.
So annotate kernel_map_range() with __ref to tell modpost to
ignore the reference to a __init function.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>