dm ima: add a warning in dm_init if duplicate ima events are not measured

The end-users of DM devices/targets may remove and re-create the same
device multiple times.  IMA does not measure such duplicate events if the
configuration CONFIG_IMA_DISABLE_HTABLE is set to 'n'.
To avoid confusion, the end-users need some indication on the client
if that configuration option is disabled.

Add a one-time warning during dm_init() if CONFIG_IMA_DISABLE_HTABLE
is set to 'n', to notify the end-users that duplicate events will not
be measured in the ima log. Also cleanup some whitespace in dm_init().

Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>