nfs: fix broken handling of the softreval mount option

Turns out that ever since this mount option was added, passing
`softreval` in NFS mount options cancelled all other flags while not
affecting the underlying flag `NFS_MOUNT_SOFTREVAL`.

Fixes: c74dfe97

 ("NFS: Add mount option 'softreval'")
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>