sparc64: Fix lmb_reserve() args in find_ramdisk().

This fixes the missing ram regression reported by
Mikael Pettersson <mikpe@it.uu.se>, much thanks for
all of this help in diagnosing this.

The second argument to lmb_reserve() is a size,
not an end address bounds.

Tested-by: Mikael Pettersson <mikpe@it.uu.se>

Signed-off-by: David S. Miller <davem@davemloft.net>