NFS: Fix a double free in nfs_parse_mount_options()

Due to an apparent typo, commit a67d18f8
(NFS: load the rpc/rdma transport module automatically) lead to the
'proto=' mount option doing a double free, while Opt_mountproto leaks a
string.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>