UBUNTU: SAUCE: (namespace) block_dev: Check permissions towards block device inode...

UBUNTU: SAUCE: (namespace) block_dev: Check permissions towards block device inode when mounting

Unprivileged users should not be able to mount block devices when
they lack sufficient privileges towards the block device inode.
Update blkdev_get_by_path() 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.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>