max_map_count: filter out not accounted vector stub for arm

Arm has following vector stub:
ffff0000-ffff1000 r-xp 00000000 00:00 0 [vectors]
which is not accounted in map_count.

There are 8 vector regions on arm, only this one is not accounted.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>