yaffs2: fix memory leak in mount/umount

when using yaffs2 filesystem, after umounting, yaffs_put_super
function doesn't free the context_os memory, it is malloced in
yaffs_internal_read_super.

unreferenced object 0xd9103980 (size 64):
  comm "mount", pid 5694, jiffies 159571 (age 45.150s)
  hex dump (first 32 bytes):
    80 39 10 d9 80 39 10 d9 00 b0 83 dc 00 b0 07 d9 .9...9..........
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  backtrace:
    [<c0131b20>] kmem_cache_alloc_trace+0x188/0x350
    [<c0386720>] yaffs_internal_read_super.isra.18+0x1d0/0x7e4
    [<c0386d58>] yaffs2_internal_read_super_mtd+0x24/0x34
    [<c0147110>] mount_bdev+0x178/0x1a0
    [<c03835c8>] yaffs2_mount+0x28/0x30
    [<c0147a74>] mount_fs+0x54/0x194
    [<c01620e0>] vfs_kern_mount+0x58/0xf0
    [<c0164b18>] do_mount+0x210/0xa48
    [<c01656a4>] SyS_mount+0x94/0xc8
    [<c000ec80>] ret_fast_syscall+0x0/0x30
    [<ffffffff>] 0xffffffff

Signed-off-by: czou <cao.zou@windriver.com>