NFSv4: Fix the referral mount code

Fix a typo which causes try_location() to use the wrong length argument
when calling nfs_parse_server_name(). This again, causes the initialisation
of the mount's sockaddr structure to fail.

Also ensure that if nfs4_pathname_string() returns an error, then we pass
that error back up the stack instead of ENOENT.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>