os/bluestore/BlueFS: use nullptr when appropriate

instead of using `0` to represent a nullptr, use nullptr for better
readability.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

src/os/bluestore/BlueFS.h

@@ -563,7 +563,7 @@ class BlueFS {
   struct {
     ceph::mutex lock = ceph::make_mutex("BlueFS::log.lock");
     uint64_t seq_live = 1;   //seq that log is currently writing to; mirrors dirty.seq_live
-    FileWriter *writer = 0;
+    FileWriter *writer = nullptr;
     bluefs_transaction_t t;
     bool uses_envelope_mode = false; // true if any file is in envelope mode
   } log;