dm ioctl: Check dm_target_spec is sufficiently aligned

Otherwise subsequent code, if given malformed input, could dereference
a misaligned 'struct dm_target_spec *'.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au> # use %zu
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>