Revert "Btrfs: device_list_add() should not update list when mounted"

Revert "Btrfs: device_list_add() should not update list when mounted"

This reverts commit bcb0513029f1721e4e28e3a87507fef053d505d7.

This commit is triggering failures to mount by subvolume id in some
configurations.  The main problem is how many different ways this
scanning function is used, both for scanning while mounted and
unmounted.  A proper cleanup is too big for late rcs.

For now, just revert the commit and we'll put a better fix into a later
merge window.

Signed-off-by: Chris Mason <clm@fb.com>