ARM: initrd: disable initrd if passed address overlaps reserved region

Disable the initrd if the passed address already overlaps the reserved
region.  This avoids oopses on Netwinders when NeTTrom tells the kernel
that an initrd is located at mem+4MB, but this overlaps the BSS,
resulting in the kernels in-use BSS being freed.

This should be applied to v2.6.37-stable.

Cc: <stable@kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>