NFS: Error out when register_shrinker fail in register_nfs_fs

Commit 1d3d4437 "vmscan: per-node deferred work" have made
register_shrinker can return an intergater error.

If register_shrinker() fail, the later unregister_shrinker() will
 cause a NULL pointer access.

v2, same as v1.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>