Unmounting the log can fail. unlikely, but it can. Catch all the error
conditions an make sure it's propagated upwards.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30833a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NNiv Sardi <xaiki@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>