loop_init doesn't fail gracefully for two reasons:
1) If initialization of loop driver fails, we have an call to
devfs_add("loop") without any devfs_remove; I add that.
2) On lwn.net 2.6 kernel docs, Jonathan Corbet says: "If you are calling
add_disk() in your driver initialization routine, you should not fail
the initialization process after the first call."
So I make loop.c conform to this request by moving add_disk after all
memory allocations.