mount: Fix handling of removed IO limiting groups

Fuse workers released IO limitng lock while waiting out the master
communication delay. After reacquiring the lock, they didn't verify
that their group hasn't been removed and sometimes sent requests for
no longer existing group.

Check group validity after lock reacquisition in such situations.

Change-Id: If149c8e0958db22941475333fc5ea7ecc09bea2f
master: Fix handling of removed IO limiting groups

There is a short race window when mount can queue an IO limiting request
for group which is being removed from the master configuration at the
exact same time.

When this happened, mount was repeatedly sending this packet and getting
disconnected by master, until the transmission retry counter has reached
its limit.

Make master respond with 0 to requests for groups which don't exist to
avoid the repeated retransmissions and disconnections.

Change-Id: Idca1b3992d454ed844bd715b1ce4024e8aff1722