f2fs: Fix double free issue for s_encoding

In case of error in fsync data recovery during mount,
sb->s_encoding is freed but not set to NULL, resulting
in double free issue in f2fs_put_super during unmount.

Log: Object already free
[ 1447.382542] INFO: Allocated in kvmalloc_node+0x58/0x9c
   age=351740 cpu=7 pid=695
[ 1447.382544] kvmalloc_node+0x58/0x9c
[ 1447.382549] f2fs_build_node_manager+0x244/0x4f0
[ 1447.382554] f2fs_fill_super+0x14f4/0x1c04
[ 1447.382557] mount_bdev+0x178/0x1d8
[ 1447.382558] f2fs_mount+0x18/0x24
[ 1447.382562] legacy_get_tree+0x4c/0xac
[ 1447.382563] vfs_get_tree+0x4c/0x118
[ 1447.382566] do_mount+0x678/0xcc4
[ 1447.382568] ksys_mount+0x98/0xdc
<>
[ 1447.382580] INFO: Freed in kvfree+0x38/0x44
  age=16 cpu=7 pid=1
[ 1447.382582] kvfree+0x38/0x44
[ 1447.382584] f2fs_destroy_node_manager+0x2f4/0x38c
[ 1447.382585] f2fs_put_super+0x198/0x2b4
[ 1447.382588] generic_shutdown_super+0x8c/0x140
[ 1447.382589] kill_block_super+0x20/0x50
[ 1447.382591] kill_f2fs_super+0xc0/0x108
[ 1447.382593] deactivate_locked_super+0x78/0x11c
[ 1447.382595] deactivate_super+0x80/0xa4
[ 1447.382596] cleanup_mnt+0x11c/0x158
[ 1447.382598] __cleanup_mnt+0x14/0x20

This change sets s_encoding to NULL after its freed
to avoid double free issue.

Change-Id: I017ba5386e76c81d7e545d3eefc2ed02dc6522cc
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>