Fix !tx_abdicate error from r336560

r336560 was supposed to restore pre-r323954 behaviour when tx_abdicate is
not set (the default case). However, it appears that rather than the drainage
check being made conditional on tx_abdicate being set, it was duplicated
so it occured twice if tx_abdicate was set and once if it was not.

Now when !tx_abdicate, drainage is only checked if the doorbell isn't
pending.

Reported by: lev
MFC after: 1 week
Sponsored by: Limelight Networks