Allocate zap_attribute_t from kmem instead of stack

This patch is preparatory work for long name feature. It changes all
users of zap_attribute_t to allocate it from kmem instead of stack. It
also make zap_attribute_t and zap_name_t structure variable length.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #15921

include/sys/zap.h

@@ -376,9 +376,19 @@ typedef struct {
 	boolean_t za_normalization_conflict;
 	uint64_t za_num_integers;
 	uint64_t za_first_integer;	/* no sign extension for <8byte ints */
-	char za_name[ZAP_MAXNAMELEN];
+	uint32_t za_name_len;
+	char za_name[];
 } zap_attribute_t;
 
+void zap_init(void);
+void zap_fini(void);
+
+/*
+ * Alloc and free zap_attribute_t.
+ */
+zap_attribute_t *zap_attribute_alloc(void);
+void zap_attribute_free(zap_attribute_t *attrp);
+
 /*
  * The interface for listing all the attributes of a zapobj can be
  * thought of as cursor moving down a list of the attributes one by