bcachefs: add check NULL return of bio_kmalloc in journal_read_bucket

bio_kmalloc may return NULL, will cause NULL pointer dereference.
Add check NULL return for bio_kmalloc in journal_read_bucket.

Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Fixes: ac10a961 ("bcachefs: Some fixes for building in userspace")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>