mm: validate inode in mapping_set_error()

The swap address_space doesn't have host. Thus, it makes kernel crash once
swap write meets error. Fix it.

Fixes: 735e4ae5 ("vfs: track per-sb writeback errors and report them to syncfs")
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Andres Freund <andres@anarazel.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201010000650.750063-1-minchan@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -54,7 +54,8 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
 	__filemap_set_wb_err(mapping, error);
 
 	/* Record it in superblock */
-	errseq_set(&mapping->host->i_sb->s_wb_err, error);
+	if (mapping->host)
+		errseq_set(&mapping->host->i_sb->s_wb_err, error);
 
 	/* Record it in flags for now, for legacy callers */
 	if (error == -ENOSPC)