[PATCH][SRU][XENIAL][YAKKETY] Fix ztest truncated cache file

[Colin King]

From: Brian Behlendorf <behlendorf1 at llnl.gov>

BugLink: http://bugs.launchpad.net/bugs/1587686

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 at llnl.gov>
Closes #4129
(cherry picked from zfs commit 151f84e2c32f690b92c424d8c55d2dfccaa76e51)
Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 zfs/module/zfs/spa_config.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/zfs/module/zfs/spa_config.c b/zfs/module/zfs/spa_config.c
index 929f181..19432e0 100644
--- a/zfs/module/zfs/spa_config.c
+++ b/zfs/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) {
-- 
2.8.1