dm/dax: Fix table reference counts

A recent fix to the dm_dax_supported() flow uncovered a latent bug. When
dm_get_live_table() fails it is still required to drop the
srcu_read_lock(). Without this change the lvm2 test-suite triggers this
warning:

    # lvm2-testsuite --only pvmove-abort-all.sh

    WARNING: lock held when returning to user space!
    5.9.0-rc5+ #251 Tainted: G           OE
    ------------------------------------------------
    lvm/1318 is leaving the kernel with locks still held!
    1 lock held by lvm/1318:
     #0: ffff9372abb5a340 (&md->io_barrier){....}-{0:0}, at: dm_get_live_table+0x5/0xb0 [dm_mod]

...and later on this hang signature:

    INFO: task lvm:1344 blocked for more than 122 seconds.
          Tainted: G           OE     5.9.0-rc5+ #251
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    task:lvm             state:D stack:    0 pid: 1344 ppid:     1 flags:0x00004000
    Call Trace:
     __schedule+0x45f/0xa80
     ? finish_task_switch+0x249/0x2c0
     ? wait_for_completion+0x86/0x110
     schedule+0x5f/0xd0
     schedule_timeout+0x212/0x2a0
     ? __schedule+0x467/0xa80
     ? wait_for_completion+0x86/0x110
     wait_for_completion+0xb0/0x110
     __synchronize_srcu+0xd1/0x160
     ? __bpf_trace_rcu_utilization+0x10/0x10
     __dm_suspend+0x6d/0x210 [dm_mod]
     dm_suspend+0xf6/0x140 [dm_mod]

Fixes: 7bf7eac8d6 ("dax: Arrange for dax_supported check to span multiple devices")
Cc: <stable@vger.kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Reported-by: Adrian Huang <ahuang12@lenovo.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Tested-by: Adrian Huang <ahuang12@lenovo.com>
Link: https://lore.kernel.org/r/160045867590.25663.7548541079217827340.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/dm.c

@@ -1136,15 +1136,16 @@ static bool dm_dax_supported(struct dax_device *dax_dev, struct block_device *bd
 {
 	struct mapped_device *md = dax_get_private(dax_dev);
 	struct dm_table *map;
+	bool ret = false;
 	int srcu_idx;
-	bool ret;
 
 	map = dm_get_live_table(md, &srcu_idx);
 	if (!map)
-		return false;
+		goto out;
 
 	ret = dm_table_supports_dax(map, device_supports_dax, &blocksize);
 
+out:
 	dm_put_live_table(md, srcu_idx);
 
 	return ret;