[XFS] don't call xfs_freesb from xfs_mountfs failure case

Freeing of the superblock is already handled in the caller, and that is
more symmetric with the mount path, too.

SGI-PV: 981951
SGI-Modid: xfs-linux-melb:xfs-kern:31192a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>