arm: mm: Check if movable zone has a nonzero size

If CONFIG_FIX_MOVABLE_ZONE is enabled but the
the movable zone is not set up, vmalloc_min
will become 0 and the kernel will crash.

This change checks that movable_reserved_size
is nonzero before doing anything.

Change-Id: I3d3cb2482fde20fbd1dd0668dbbd414e42eda995
Signed-off-by: Jack Cheung <jackc@codeaurora.org>