btrfs: unify checking of IS_ERR and null

use IS_ERR_OR_NULL when possible, done by this coccinelle script:

@ match @
identifier id;
@@
(
- BUG_ON(IS_ERR(id) || !id);
+ BUG_ON(IS_ERR_OR_NULL(id));
|
- IS_ERR(id) || !id
+ IS_ERR_OR_NULL(id)
|
- !id || IS_ERR(id)
+ IS_ERR_OR_NULL(id)
)

Signed-off-by: David Sterba <dsterba@suse.cz>