f2fs: fix null pointer panic in tracepoint in __replace_atomic_write_block

commit da6ea0b0 upstream.

We got a kernel panic if old_addr is NULL.

https://bugzilla.kernel.org/show_bug.cgi?id=217266

BUG: kernel NULL pointer dereference, address: 0000000000000000
 Call Trace:
  <TASK>
  f2fs_commit_atomic_write+0x619/0x990 [f2fs a1b985b80f5babd6f3ea778384908880812bfa43]
  __f2fs_ioctl+0xd8e/0x4080 [f2fs a1b985b80f5babd6f3ea778384908880812bfa43]
  ? vfs_write+0x2ae/0x3f0
  ? vfs_write+0x2ae/0x3f0
  __x64_sys_ioctl+0x91/0xd0
  do_syscall_64+0x5c/0x90
  entry_SYSCALL_64_after_hwframe+0x72/0xdc
 RIP: 0033:0x7f69095fe53f

Fixes: 2f3a9ae9

 ("f2fs: introduce trace_f2fs_replace_atomic_write_block")
Cc: <stable@vger.kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>