libxfs: refactor manage_zones()

libxfs: refactor manage_zones()

It's bizarre to have manage_zones() both set up and tear down zones.
It's even more bizarre to have xfs_dir_startup() buried in there.

Refactor init/destory into 2 functions, and call xfs_dir_startup()
separately from zone init similar to what kernelspace does.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>