NFS: Allow folio migration for the case of mode == MIGRATE_SYNC

When the mode is MIGRATE_SYNC, we are allowed to call nfs_wb_folio()
under the folio lock.

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>