logfs: check for the return value after calling find_or_create_page()

In get_mapping_page(), after calling find_or_create_page(), the return
value should be checked.

This patch has been provided:
http://www.spinics.net/lists/linux-fsdevel/msg66948.html but not been
applied now.

Signed-off-by: Younger Liu <[e-mail]>
Cc: Younger Liu <[e-mail]>
Cc: Vyacheslav Dubeyko <[e-mail]>
Reviewed-by: Prasad Joshi <[e-mail]>
Cc: Jörn Engel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/logfs/segment.c

···
 		page = read_cache_page(mapping, index, filler, sb);
 	else {
 		page = find_or_create_page(mapping, index, GFP_NOFS);
-		unlock_page(page);
+		if (page)
+			unlock_page(page);
 	}
 	return page;
 }