NFS: Extend rdirplus mount option with "force|none"

NFS: Extend rdirplus mount option with "force|none"

There are certain users that wish to force the NFS client to choose
READDIRPLUS over READDIR for a particular mount.  Update the "rdirplus" mount
option to optionally accept values.  For "rdirplus=force", the NFS client
will always attempt to use READDDIRPLUS.  The setting of "rdirplus=none" is
aliased to the existing "nordirplus".

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Link: https://lore.kernel.org/r/c4cf0de4c8be0930b91bc74bee310d289781cd3b.1741885071.git.bcodding@redhat.com
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>