fix: missing 'lttng_fs_info_fsid' wrapper in btrfs backport

fix: missing 'lttng_fs_info_fsid' wrapper in btrfs backport

The following backported patch was missing the use of the
'lttng_fs_info_fsid' wrapper to enable compatibility with upstream
stable kernels.

  commit 065436fd07d58a9a17427ad386ffcc8d29e25a9d
  Author: Michael Jeanson <mjeanson@efficios.com>
  Date:   Mon Jun 18 14:53:18 2018 -0400

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