[JFFS2] Fix return value from jffs2_write_end()

jffs2_write_end() is sometimes passing back a "written" length greater 
than the length we passed into it, leading to a BUG at mm/filemap.c:1749 
when used with unionfs.

It happens because we actually write more than was requested, to reduce 
log fragmentation. These "longer" writes are fine, but they shouldn't 
get propagated back to the vm/vfs.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

fs/jffs2/file.c

@@ -255,7 +255,7 @@ static int jffs2_write_end(struct file *filp, struct address_space *mapping,
 		   _whole_ page. This helps to reduce the number of
 		   nodes in files which have many short writes, like
 		   syslog files. */
-		start = aligned_start = 0;
+		aligned_start = 0;
 	}
 
 	ri = jffs2_alloc_raw_inode();
@@ -291,14 +291,11 @@ static int jffs2_write_end(struct file *filp, struct address_space *mapping,
 	}
 
 	/* Adjust writtenlen for the padding we did, so we don't confuse our caller */
-	if (writtenlen < (start&3))
-		writtenlen = 0;
-	else
-		writtenlen -= (start&3);
+	writtenlen -= min(writtenlen, (start - aligned_start));
 
 	if (writtenlen) {
-		if (inode->i_size < (pg->index << PAGE_CACHE_SHIFT) + start + writtenlen) {
-			inode->i_size = (pg->index << PAGE_CACHE_SHIFT) + start + writtenlen;
+		if (inode->i_size < pos + writtenlen) {
+			inode->i_size = pos + writtenlen;
 			inode->i_blocks = (inode->i_size + 511) >> 9;
 
 			inode->i_ctime = inode->i_mtime = ITIME(je32_to_cpu(ri->ctime));