btrfs-progs: don't write memory after sb to disk

btrfs-progs: don't write memory after sb to disk

struct btrfs_super is about 3.5k but a few writing paths were writing it
out as the full 4k BTRFS_SUPER_INFO_SIZE, leaking a few hundred bytes
after the super_block onto disk.  In practice this meant the memory
after super_copy in struct btrfs_fs_info and whatever came after it in
the heap.

Signed-off-by: Zach Brown <zab@redhat.com>