imsm: save checkpoint prior to exit

imsm: save checkpoint prior to exit

If reshape (eg. chunksize migration) is gracefully stopped via SIGTERM
the checkpoint is not saved and reshape cannot be resumed due to "data
being present in copy area". This is because UNIT_SRC_NORMAL isn't set
if SIGTERM occurred.

Move SIGTERM handling at the end of the loop to allow saving checkpoint
(and state) so reshapes can be properly resumed.

Signed-off-by: Mateusz Kusiak <mateusz.kusiak@intel.com>