btrfs: remove BUG_ON(ret) in alloc_reserved_tree_block

Switch this to an ASSERT() and return the error in the normal case.

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