dm ioctl: assert _hash_lock is held in __hash_remove

Also update dm_early_create() to take _hash_lock when calling both
__get_name_cell and __hash_remove -- given dm_early_create()'s early
boot usecase this locking isn't about correctness but it allows
lockdep_assert_held() to be added to __hash_remove.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>