nfs: Remove redundant null checks before kfree

Fix the following coccicheck warning:
fs/nfs/dir.c:2494:2-7: WARNING:
NULL check before some freeing functions is not needed.

Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>