mnt: Fix validation of dumpable mountpoints

This patch consists of 3 unsplittable (from my POV) fixes.

1. Remove messy check from dump_one_mountpoint() -- we have
   validate_mounts to check whether we can dump the tree
   or not.

2. Other than being in the wron place the mentioned check
   is wrong. Comparing of the length of the mp->source-s
   makes no sense -- it should be mp->root, but even this
   would be wrong...

3. ... instead, we should check for bind mount root path
   being accessible from the target mount root path, i.e.
   the bind->root should start with src->root.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>