ARM: Fix sparsemem with SPARSEMEM_EXTREME enabled

When SPARSEMEM_EXTREME is enabled, memory_present() wants to use bootmem
to allocate data structures.  However, we call memory_present() after
declaring memory to bootmem, but before we've reserved areas.

This leads to sparsemem data structures being overwritten later in the
kernel's initialization (when slab initializes.)

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