NFS: check if suid/sgid was cleared after a write as needed

I noticed xfstests generic/193 and generic/355 started failing against
knfsd after commit e7a8ebc3 ("NFSD: Offer write delegation for OPEN
with OPEN4_SHARE_ACCESS_WRITE").

I ran those same tests against ONTAP (which has had write delegation
support for a lot longer than knfsd) and they fail there too... so
while it's a new failure against knfsd, it isn't an entirely new
failure.

Add the NFS_INO_REVAL_FORCED flag so that the presence of a delegation
doesn't keep the inode from being revalidated to fetch the updated mode.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>