crimson/os/seastore/record_scanner: replace [=] capturing

Capturing with = is error prone and causes compile warning.

And it is wrong:
* cursor should be captured by reference
* mutable keyword is unnecessary

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>

Author: cyx1231st

src/crimson/os/seastore/record_scanner.cc

@@ -120,7 +120,7 @@ RecordScanner::read_validate_record_metadata(
   }
   TRACE("reading record group header block {}~4096", start);
   return read(start, block_size
-  ).safe_then([=](bufferptr bptr) mutable
+  ).safe_then([this, FNAME, nonce, block_size, &cursor](bufferptr bptr)
               -> read_validate_record_metadata_ret {
     bufferlist bl;
     bl.append(bptr);