ima: Use i_version only when filesystem supports it

i_version is only supported by a filesystem when the SB_I_VERSION
flag is set. This patch tests for the SB_I_VERSION flag before using
i_version. If we can't use i_version to detect a file change then we
must assume the file has changed in the last_writer path and remeasure
it.

On filesystems without i_version support IMA used to measure a file
only once and didn't detect any changes to a file. With this patch
IMA now works properly on these filesystems.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

security/integrity/ima/ima_main.c

@@ -127,7 +127,8 @@ static void ima_check_last_writer(struct integrity_iint_cache *iint,
 
 	inode_lock(inode);
 	if (atomic_read(&inode->i_writecount) == 1) {
-		if ((iint->version != inode->i_version) ||
+		if (!IS_I_VERSION(inode) ||
+		    (iint->version != inode->i_version) ||
 		    (iint->flags & IMA_NEW_FILE)) {
 			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
 			iint->measured_pcrs = 0;