nfs: fix broken handling of the softreval mount option

nfs: fix broken handling of the softreval mount option

[ Upstream commit b7c0a9b0a29eacb6e06647c233925e447155995d ]

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: d54be35c0426 ("NFS: Add mount option 'softreval'")
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>