NFS: Clean up error handling in nfs_get_sb

The error return logic in nfs_get_sb now matches nfs4_get_sb, and is more maintainable.
A subsequent patch will take advantage of this simplification.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>