ARM: mm: clean up membank size limit checks

This patch cleans up the highmem sanity check code by simplifying the range
checks with a pre-calculated size_limit.  This patch should otherwise have no
functional impact on behavior.

This patch also removes a redundant (bank->start < vmalloc_limit) check, since
this is already covered by the !highmem condition.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Subash Patel <subash.rp@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>