btrfs: assert we join log transaction at btrfs_del_dir_entries_in_log()

btrfs: assert we join log transaction at btrfs_del_dir_entries_in_log()

We are supposed to be able to join a log transaction at that point, since
we have determined that the inode was logged in the current transaction
with the call to inode_logged(). So ASSERT() we joined a log transaction
and also warn if we didn't in case assertions are disabled (the kernel
config doesn't have CONFIG_BTRFS_ASSERT=y), so that the issue gets noticed
and reported if it ever happens.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>