ARM: move VMALLOC_END down temporarily for shmobile

THIS IS A TEMPORARY HACK.  The purpose of this is _only_ to avoid a
regression on an existing machine while a better fix is implemented.

On shmobile the consistent DMA memory area was set to 158MB in commit
28f0721a with no explanation.  The documented size for this area should
vary between 2MB and 14MB, and none of the other ARM targets exceed that.

The included #warning is therefore meant to be noisy on purpose to get
shmobile maintainers attention and this commit reverted once this
consistent DMA size conflict is resolved.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Magnus Damm <damm@opensource.se>
Cc: Paul Mundt <lethal@linux-sh.org>