x86, mm: Move reserving low memory later in initialization

Move the reservation of low memory, except for the 4K which actually
does belong to the BIOS, later in the initialization; in particular,
after we have already reserved the trampoline.

The current code locates the trampoline as high as possible, so by
deferring the allocation we will still be able to reserve as much
memory as is possible.  This allows us to run with reservelow=640k
without getting a crash on system startup.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/n/tip-0y9dqmmsousf69wutxwl3kkf@git.kernel.org