mds: account for header size during omap commit

fnode_t is set in the omap header during directory commit
operation which isn't accounted when tracking operation
size.

Fixes: http://tracker.ceph.com/issues/67597
Signed-off-by: Venky Shankar <vshankar@redhat.com>

src/mds/CDir.cc

@@ -2494,6 +2494,10 @@ void CDir::_omap_commit_ops(int r, int op_prio, int64_t metapool, version_t vers
       mdcache->mds->heartbeat_reset();
   }
 
+  // the last omap commit includes the omap header, so account for
+  // that size early on so that when we reach `commit_one(true)`,
+  // there is enough space for the header.
+  write_size += sizeof(fnode_t);
   using ceph::encode;
   for (auto &item : to_set) {
     bufferlist bl;