xfs_unmount is small and already pretty Linux specific, so merge it into
the callers. The real unmount path is simplified a little by doing a
WARN_ON on the xfs_unmount_flush retval directly instead of propagating
the error back to the caller, and the mout failure case in simplified
significantly by removing the forced shutdown case and all the dmapi
events that shouldn't be sent because the dmapi mount event hasn't been
sent by that time either.

SGI-PV: 981951
SGI-Modid: xfs-linux-melb:xfs-kern:31188a
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>