dm thin metadata: fix superblock refcount leak on snapshot shadow failure

dm thin metadata: fix superblock refcount leak on snapshot shadow failure

__reserve_metadata_snap() increments THIN_SUPERBLOCK_LOCATION in the
metadata space map before shadowing it. When dm_tm_shadow_block()
fails, a reference is leaked in the metadata space map.

Fix by adding the missing dm_sm_dec_block().

Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: cc8394d86f04 ("dm thin: provide userspace access to pool metadata")
Cc: stable@vger.kernel.org