f2fs/trace: fix unused variable warning

This an upstream issue and looks a bit tricky to fix but since we check for warnings, let's track this build issue:

fs/f2fs/trace.c: In function ‘f2fs_trace_pid’:
fs/f2fs/trace.c:57:16: warning: unused variable ‘inode’ [-Wunused-variable]
  struct inode *inode = page->mapping->host;
                ^~~~~

edit: I thought of a different issue from the past. this is just a bug in our downstream stuff I'll fix