btrfs: remove redundant refcount check in btrfs_put_transaction()

btrfs: remove redundant refcount check in btrfs_put_transaction()

Eric Dumazet removed the redundant refcount check for sk_refcnt, I
noticed a similar issue in btrfs_put_transaction().
refcount_dec_and_test() already checks for a zero refcount and
complains, making the preceding WARN_ON redundant. This is a leftover
from the atomic_t times.

Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>