btrfs-progs: extent_io: Init eb->lru to avoid NULL pointer dereference

btrfs-progs: extent_io: Init eb->lru to avoid NULL pointer dereference

eb->lru is not initialized in __alloc_extent_buffer(), so in the
following call chain, it could call NULL pointer dereference:

btrfs_clone_extent_buffer()
|- __alloc_extent_buffer()
   |- Now eb->lru is NULL (not initialized)

free_extent_buffer_final()
|- list_del_init(&eb->lru)

Thankfully, current btrfs-progs won't trigger such bug as the only
btrfs_clone_extent_buffer() user is paths_from_inode(), which is not
used by anyone.
(But due to the usefulness of that function in future offline scrub, I'd
like to keep this dead code.)

Anyway, initialize eb->lru in __alloc_extent_bufer() bring no harm.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>