metadump: Fill attribute values with 'v' rather than NUL

metadump: Fill attribute values with 'v' rather than NUL

Rather than memset attribute values to '\0', use the character 'v' -
otherwise in some cases we get attributes with a non-zero value
length which start with a NUL, and that makes some userspace tools
unhappy, yielding results like this:

security.oO^Lio.=0sAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>