Linux SPL module init: Handle memory allocation failures correctly

Upon inspection of our code, I noticed that we assume that
__alloc_percpu() cannot fail, and while it probably never has failed in
practice, technically, it can fail, so we should handle that.

Additionally, we incorrectly assume that `taskq_create()` in
spl_kmem_cache_init() cannot fail. The same remark applies to it.

Lastly, `spl-init()` failures should always return negative error
values, but in some places, we are returning positive 1, which is
incorrect. We change those values to their correct error codes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13847

module/os/linux/spl/spl-kmem-cache.c

@@ -1452,6 +1452,9 @@ spl_kmem_cache_init(void)
 	    spl_kmem_cache_kmem_threads * 8, INT_MAX,
 	    TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 
+	if (spl_kmem_cache_taskq == NULL)
+		return (-ENOMEM);
+
 	return (0);
 }