cifs: fix string declarations and assignments in tracepoints

We missed using the variable length string macros in several
tracepoints. Fixed them in this change.

There's probably more useful macros that we can use to print
others like flags etc. But I'll submit sepawrate patches for
those at a future date.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Cc: <stable@vger.kernel.org> # v5.12
Signed-off-by: Steve French <stfrench@microsoft.com>