nfs4: flex_file: ignore synthetic uid/gid for tightly coupled DSes

for tightly coupled DSes client must ignore provided synthetic uid and
gid as stated in draft-ietf-nfsv4-flex-files-19#section-5.1.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>