btrfs: prefer IS_ERR_OR_NULL() over manual NULL check

btrfs: prefer IS_ERR_OR_NULL() over manual NULL check

Prefer using IS_ERR_OR_NULL() over using IS_ERR() and a manual NULL
check.

IS_ERR_OR_NULL() already uses likely(!ptr) internally. checkpatch does
not like nesting it:
> WARNING: nested (un)?likely() calls, IS_ERR_OR_NULL already uses
> unlikely() internally
Remove the explicit use of likely().

Change generated with coccinelle.

Signed-off-by: Philipp Hahn <phahn-oss@avm.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>