xfs: use private fs freeze for fscounters fsck

Now that we've eliminated all the other online fsck users that invoke a
VFS freeze, convert the sole remaining caller (summary counters) to use
a private version of the VFS freeze routines.  This constrains all that
craziness to a single file and enables us to take an 'exclusive' freeze
that prevents anyone else from wandering in and thawing the fs.
-----BEGIN PGP SIGNATURE-----
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=lsw/
-----END PGP SIGNATURE-----