btrfs: print a message when a missing device re-appears

btrfs: print a message when a missing device re-appears

There is a bug report that fstrim crashed, and that crash is eventually
pinned down to a missing device which re-appeared and screwed up callers
that only checks BTRFS_DEV_STATE_MISSING, but not
BTRFS_DEV_STATE_WRITEABLE nor device->bdev.

A missing device re-appearing can be very tricky, as for now it will
result in a device without WRITEABLE or MISSING flag, and still no bdev
pointer.

As the first step to enhance handling of such re-appearing missing
devices, add a dmesg output when a missing device re-appeared.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>