jffs2: fix memory leak in jffs2_do_mount_fs

If jffs2_build_filesystem() in jffs2_do_mount_fs() returns an error,
we can observe the following kmemleak report:

--------------------------------------------
unreferenced object 0xffff88811b25a640 (size 64):
  comm "mount", pid 691, jiffies 4294957728 (age 71.952s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffffa493be24>] kmem_cache_alloc_trace+0x584/0x880
    [<ffffffffa5423a06>] jffs2_sum_init+0x86/0x130
    [<ffffffffa5400e58>] jffs2_do_mount_fs+0x798/0xac0
    [<ffffffffa540acf3>] jffs2_do_fill_super+0x383/0xc30
    [<ffffffffa540c00a>] jffs2_fill_super+0x2ea/0x4c0
    [...]
unreferenced object 0xffff88812c760000 (size 65536):
  comm "mount", pid 691, jiffies 4294957728 (age 71.952s)
  hex dump (first 32 bytes):
    bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
    bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
  backtrace:
    [<ffffffffa493a449>] __kmalloc+0x6b9/0x910
    [<ffffffffa5423a57>] jffs2_sum_init+0xd7/0x130
    [<ffffffffa5400e58>] jffs2_do_mount_fs+0x798/0xac0
    [<ffffffffa540acf3>] jffs2_do_fill_super+0x383/0xc30
    [<ffffffffa540c00a>] jffs2_fill_super+0x2ea/0x4c0
    [...]
--------------------------------------------

This is because the resources allocated in jffs2_sum_init() are not
released. Call jffs2_sum_exit() to release these resources to solve
the problem.

Fixes: e631ddba ("[JFFS2] Add erase block summary support (mount time improvement)")
Cc: stable@vger.kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>