fs,nfs: lift compat nfs4 mount data handling into the nfs code

There is no reason the generic fs code should bother with NFS specific
binary mount data - lift the conversion into nfs4_parse_monolithic
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>