UPSTREAM: ARM: poison the vectors page

(cherry pick from commit f928d4f2)

Fill the empty regions of the vectors page with an exception generating
instruction.  This ensures that any inappropriate branch to the vector
page is appropriately trapped, rather than just encountering some code
to execute.  (The vectors page was filled with zero before, which
corresponds with the "andeq r0, r0, r0" instruction - a no-op.)

Cc: <stable@vger.kernel.org>
Acked-by Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Bug: 9674955
Bug: 25773781
Change-Id: I1a68ca56922f0b3af9b299fe53b3ffd255309f02