Fix: btrfs: use fs_info for btrfs_handle_em_exist tracepoint

See upstream commit:

  commit f46b24c9457143a367c6707eac82d546e2bcf280
  Author: David Sterba <dsterba@suse.com>
  Date:   Tue Apr 3 21:45:57 2018 +0200

    btrfs: use fs_info for btrfs_handle_em_exist tracepoint

    We really want to know to which filesystem the extent map events belong,
    but as it cannot be reached from the extent_map pointers, we need to
    pass it down the callchain.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>