jffs2: Fix memory leak in jffs2_scan_eraseblock() error path

In jffs2_scan_eraseblock(), 'sumptr' is allocated through kmalloc() if
'sumlen' is larger than 'buf_size'. However, it is not deallocated in the
following execution if jffs2_fill_scan_buf() fails, leading to a memory
leak bug. To fix this issue, free 'sumptr' before returning the error.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Richard Weinberger <richard@nod.at>