libxfs: fix memory leasks in libxfs_umount()

libxfs: fix memory leasks in libxfs_umount()

libxfs_umount was failing to free a handful of resources; fix that
up.  Call it from xfs_copy as well, while we're at it; every other
libxfs_mount has a libxfs_umount counterpart, at least on a clean
exit.

[dchinner: fix superblock buffer leak uncovered by adding
 libxfs_umount() to xfs_copy. ]

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>