mm: fix mapping_set_error call in me_pagecache_dirty

The error code should be negative.  Since this ends up in the default case
anyway, this is harmless, but it's less confusing to negate it.  Also,
later patches will require a negative error code here.

Link: http://lkml.kernel.org/r/20170525103355.6760-1-jlayton@redhat.com
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>