btrfs: Use list_empty instead of list_empty_careful

list_empty_careful usually is a signal of something tricky going on. Its
usage in btrfs is actually not needed since both lists it's used on are
local to a function and cannot be modified concurrently. So switch to
plain list_empty. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>