Adjust the vfs_open to provide cred variable with 4.18 kernels

Adjust the vfs_open to provide cred variable with 4.18 kernels

The kernel's git commit ae2bb293a3e8adbc54d08cede5afc22929030c03
removed the cred argument from the vfs_open.  Thus, there is no $cred
target variable available.  This missing target variable lives on as a
field in the $file target variable.  The patch makes the tapset use
that field if the $cred target variable is not available.  Fixing this
allows the slowvfs.stp example to work with newer linux 4.18 kernels.