UBI: add ioctl for is_mapped operation

This patch adds ioctl to check if an LEB is mapped or not (as a
debugging option so far).

[Re-named ioctl to make it look the same as the other one and made
 some minor stylistic changes. Artem Bityutskiy.]

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>