UBUNTU: SAUCE: mtd: Check permissions towards mtd block device inode when mounting

UBUNTU: SAUCE: mtd: Check permissions towards mtd block device inode when mounting

Unprivileged users should not be able to mount mtd block devices
when they lack sufficient privileges towards the block device
inode.  Update mount_mtd() to validate that the user has the
required access to the inode at the specified path. The check
will be skipped for CAP_SYS_ADMIN, so privileged mounts will
continue working as before.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>