os/bluestore: ensure block device size is a multiple of the block size

We might have a backing device that is an odd number of 512-byte sectors
but have the block_size configured to 4096.  Ensure the reported size
rounds down to avoid confusing other layers of the stack.

Signed-off-by: Sage Weil <sage@redhat.com>

src/os/bluestore/KernelDevice.cc

@@ -140,6 +140,9 @@ int KernelDevice::open(string p)
   fs = FS::create_by_fd(fd_direct);
   assert(fs);
 
+  // round size down to an even block
+  size &= ~(block_size - 1);
+
   r = _aio_start();
   assert(r == 0);
 

src/os/bluestore/NVMEDevice.cc

@@ -813,6 +813,9 @@ int NVMEDevice::open(string p)
   //nvme is non-rotational device.
   rotational = false;
 
+  // round size down to an even block
+  size &= ~(block_size - 1);
+
   dout(1) << __func__ << " size " << size << " (" << pretty_si_t(size) << "B)"
           << " block_size " << block_size << " (" << pretty_si_t(block_size)
           << "B)" << dendl;