storage: Check FS pool source during virStorageBackendFileSystemIsMounted

https://bugzilla.redhat.com/show_bug.cgi?id=1025230

When determining whether a FS pool is mounted, rather than assuming that
the FS pool is mounted just because the target.path is in the mount list,
let's make sure that the FS pool source matches what is mounted

src/storage/storage_backend_fs.c

@@ -419,6 +419,7 @@ static int
 virStorageBackendFileSystemIsMounted(virStoragePoolObjPtr pool)
 {
     int ret = -1;
+    char *src = NULL;
     FILE *mtab;
     struct mntent ent;
     char buf[1024];
@@ -431,16 +432,23 @@ virStorageBackendFileSystemIsMounted(virStoragePoolObjPtr pool)
     }
 
     while ((getmntent_r(mtab, &ent, buf, sizeof(buf))) != NULL) {
-        if (STREQ(ent.mnt_dir, pool->def->target.path)) {
+        if (!(src = virStorageBackendFileSystemGetPoolSource(pool)))
+            goto cleanup;
+
+        if (STREQ(ent.mnt_dir, pool->def->target.path) &&
+            STREQ(ent.mnt_fsname, src)) {
             ret = 1;
             goto cleanup;
         }
+
+        VIR_FREE(src);
     }
 
     ret = 0;
 
  cleanup:
     VIR_FORCE_FCLOSE(mtab);
+    VIR_FREE(src);
     return ret;
 }