btrfs: handle errors from select_reloc_root()

Currently select_reloc_root() doesn't return an error, but followup
patches will make it possible for it to return an error.  We do have
proper error recovery in do_relocation however, so handle the
possibility of select_reloc_root() having an error properly instead of
BUG_ON(!root).

I've also adjusted select_reloc_root() to return ERR_PTR(-ENOENT) if we
don't find a root, instead of NULL, to make the error case easier to
deal with.  I've replaced the BUG_ON(!root) with an ASSERT(0) for this
case as it indicates we messed up the backref walking code, but it could
also indicate corruption.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>