Fix ztest truncated cache file

Commit efc412b updated spa_config_write() for Linux 4.2 kernels to
truncate and overwrite rather than rename the cache file.  This is
the correct fix but it should have only been applied for the kernel
build.  In user space rename(2) is needed because ztest depends on
the cache file.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4129

module/zfs/spa_config.c

@@ -174,7 +174,7 @@ spa_config_write(spa_config_dirent_t *dp, nvlist_t *nvl)
 	VERIFY(nvlist_pack(nvl, &buf, &buflen, NV_ENCODE_XDR,
 	    KM_SLEEP) == 0);
 
-#ifdef __linux__
+#if defined(__linux__) && defined(_KERNEL)
 	/*
 	 * Write the configuration to disk.  Due to the complexity involved
 	 * in performing a rename from within the kernel the file is truncated
@@ -201,7 +201,8 @@ spa_config_write(spa_config_dirent_t *dp, nvlist_t *nvl)
 	 */
 	(void) snprintf(temp, MAXPATHLEN, "%s.tmp", dp->scd_path);
 
-	if (vn_open(temp, UIO_SYSSPACE, oflags, 0644, &vp, CRCREAT, 0) == 0) {
+	error = vn_open(temp, UIO_SYSSPACE, oflags, 0644, &vp, CRCREAT, 0);
+	if (error == 0) {
 		if (vn_rdwr(UIO_WRITE, vp, buf, buflen, 0, UIO_SYSSPACE,
 		    0, RLIM64_INFINITY, kcred, NULL) == 0 &&
 		    VOP_FSYNC(vp, FSYNC, kcred, NULL) == 0) {