uncrypt: Fix the comparison in FindBlockDevice().

Previously it considered a match if the given path (i.e. path to an
update package) fully equals to a mount_point.  For example, `uncrypt
/data block.map` or `uncrypt /vendor block.map` would exit successfully,
without producing a block map.

Test: `uncrypt /path/to/package.zip block.map`
Test: `uncrypt /vendor block.map` fails.
Change-Id: Id946ab1c0b158b623013f89463cbb1960141d8b5