Fixing issue with bit allocation byteoffset calculation

The byteoffset calculation was skewed to double include
the offset value calculated. The double calculation
happens if the starting ordinal is part of the head
sequence block. This error in calculation could result
in duplicate but getting allocated eventually propogating
to ipam and vni id allocations

Signed-off-by: Abhinandan Prativadi <abhi@docker.com>

bitseq/sequence.go

@@ -350,7 +350,6 @@ func (h *Handle) set(ordinal, start, end uint64, any bool, release bool) (uint64
 		} else {
 			nh.unselected--
 		}
-
 		// Attempt to write private copy to store
 		if err := nh.writeToStore(); err != nil {
 			if _, ok := err.(types.RetryError); !ok {
@@ -492,22 +491,27 @@ func (h *Handle) UnmarshalJSON(data []byte) error {
 
 // getFirstAvailable looks for the first unset bit in passed mask starting from start
 func getFirstAvailable(head *sequence, start uint64) (uint64, uint64, error) {
+
 	// Find sequence which contains the start bit
 	byteStart, bitStart := ordinalToPos(start)
 	current, _, _, inBlockBytePos := findSequence(head, byteStart)
 
 	// Derive the this sequence offsets
 	byteOffset := byteStart - inBlockBytePos
 	bitOffset := inBlockBytePos*8 + bitStart
-
+	var firstOffset uint64
+	if current == head {
+		firstOffset = byteOffset
+	}
 	for current != nil {
 		if current.block != blockMAX {
 			bytePos, bitPos, err := current.getAvailableBit(bitOffset)
 			return byteOffset + bytePos, bitPos, err
 		}
 		// Moving to next block: Reset bit offset.
 		bitOffset = 0
-		byteOffset += current.count * blockBytes
+		byteOffset += (current.count * blockBytes) - firstOffset
+		firstOffset = 0
 		current = current.next
 	}
 	return invalidPos, invalidPos, ErrNoBitAvailable
@@ -524,6 +528,7 @@ func getAvailableFromCurrent(head *sequence, start, curr, end uint64) (uint64, u
 		if end < ret {
 			goto begin
 		}
+
 		return bytePos, bitPos, nil
 	}