arm64: mm: only perform memstart_addr sanity check if DEBUG_VM

arm64: mm: only perform memstart_addr sanity check if DEBUG_VM

Checking whether memstart_addr has been assigned every time it is
referenced adds a branch instruction that may hurt performance if
the reference in question occurs on a hot path. So only perform the
check if CONFIG_DEBUG_VM=y.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[catalin.marinas@arm.com: replaced #ifdef with VM_BUG_ON]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit a92405f082d43267575444a6927085e4c8a69e4e)
Signed-off-by: Alex Shi <alex.shi@linaro.org>