os/bluestore: add support for allocating 0-sized extent

For testing, we may choose randomized length of extent to allocate,
which means we could trim down the length to 0.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/os/bluestore/BitMapAllocator.cc

@@ -105,6 +105,8 @@ int BitMapAllocator::allocate(
   uint64_t want_size, uint64_t alloc_unit, int64_t hint,
   uint64_t *offset, uint32_t *length)
 {
+  if (want_size == 0)
+    return 0;
 
   assert(!(alloc_unit % m_block_size));
   int64_t nblks = (want_size + m_block_size - 1) / m_block_size;