NFSv4.1: Ensure we clear the SP4_MACH_CRED flags in nfs4_sp4_select_mode()

If the server changes, so that it no longer supports SP4_MACH_CRED, or
that it doesn't support the same set of SP4_MACH_CRED functionality,
then we want to ensure that we clear the unsupported flags.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>