[XFS] don't leak m_fsname/m_rtname/m_logname

Add a helper to free the m_fsname/m_rtname/m_logname allocations and use
it properly for all mount failure cases. Also switch the allocations for
these to kstrdup while we're at it.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31728a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>