xfs: cleanup log unmount handling

Kill the current xfs_log_unmount wrapper and opencode the two function
calls in the only caller.  Rename the current xfs_log_unmount_dealloc to
xfs_log_unmount as it undoes xfs_log_mount and the new name makes that
more clear.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>