nfsd: rework NFS4_SHARE_WANT_* flag handling

The delstid draft adds new NFS4_SHARE_WANT_TYPE_MASK values that don't
fit neatly into the existing WANT_MASK or WHEN_MASK. Add a new
NFS4_SHARE_WANT_MOD_MASK value and redefine NFS4_SHARE_WANT_MASK to
include it.

Also fix the checks in nfsd4_deleg_xgrade_none_ext() to check for the
flags instead of equality, since there may be modifier flags in the
value.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>