qa: wait for file creation before changing mode

Fixes: http://tracker.ceph.com/issues/67408
Signed-off-by: Venky Shankar <[email protected]>

Author: vshankar

qa/workunits/fs/snaps/snaptest-double-null.sh

@@ -11,6 +11,7 @@ mkdir a
 cat > a/foo &
 mkdir a/.snap/one
 mkdir a/.snap/two
+wait
 chmod 777 a/foo
 sync   # this might crash the mds
 ps