test/maps01: document mmap() failures

Arm32 devices often have poor amount of RAM. E.g., RPI2 board
has only 1Gb memory. This lead to failures with maps01 test,
which tries to mmap() 1Gb of memory. The failures of test is
not very evident - let's document mmap() errors here to
blow some light on the situation.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>