btrfs-progs: skip mounted fs when deal with umounted ones for fi show

btrfs-progs: skip mounted fs when deal with umounted ones for fi show

Stalling problems may happen when exec balance & fi show cmds concurrently.

With the following commit:
        commit 915902c500
        btrfs-progs: fix device missing of btrfs fi show with seed devices

The fi show cmd will bother the mounted fs when only umounted fs should
be handled after @btrfs_can_kernel() has finished showing all mounted ones.

We could skip the mounted fs after @btrfs_can_kernel() is done, then tasks
keeps going on mounted fs while fi show continues on umounted ones separately.

Reported-by: Petr Janecek <janecek@ucw.cz>
Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>