[SPARC64]: fix section mismatch warning in mdesc.c

Fix following warning:
WARNING: vmlinux.o(.text+0x35264): Section mismatch: reference to .init.text:__alloc_bootmem (between 'mdesc_bootmem_alloc' and 'mdesc_bootmem_free')

Rename mdesc_mem_ops to *_ops so modpost ignores __init references
and declare mdesc_bootmem_alloc __init since it is only used
during __init context.

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