btrfs: increase ctx->pos for delayed dir index

Our dir_context->pos is supposed to hold the next position we're
supposed to look.  If we successfully insert a delayed dir index we
could end up with a duplicate entry because we don't increase ctx->pos
after doing the dir_emit.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>