Fix vfs.open probe use of 'cred' variable

Fix vfs.open probe use of 'cred' variable

Commit f6d683666 adjusted the vfs.open probe to obtain the $cred
target variable from a field of the $file target variable on
kernels where $cred is not available.  This change missed a
second reference to the $cred target variable within the probe.

Fix the reference to the possibly-unavailable target variable
by referencing the systemtap variable just obtained.

Fixes: Commit f6d683666 ("Adjust the vfs_open to provide cred variable with 4.18 kernels")
Signed-off-by: Frank Sorenson <sorenson@redhat.com>