cephfs: change the permissions of open files in file_test.go
cephfs: change the permissions of open files in file_test.go

As pointed out in a review of Pwritev/Preadv functions, someone could
be using the tests as example code and not realize the "wide open"
permissions used in the test. Because it doesn't actually impact the
behavior of the test there's no reason not use use more restricted
permissions.
This change makes the older functions in the file use the restricted
perms too.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph