NFS: Clean up cache validity checking

Consolidate the open-coded checking of NFS_I(inode)->cache_validity
into a couple of helper functions.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 6b79c2c..09ca5095 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -381,6 +381,7 @@
 extern void nfs_clear_inode(struct inode *);
 extern void nfs_evict_inode(struct inode *);
 void nfs_zap_acl_cache(struct inode *inode);
+extern bool nfs_check_cache_invalid(struct inode *, unsigned long);
 extern int nfs_wait_bit_killable(struct wait_bit_key *key, int mode);
 extern int nfs_wait_atomic_killable(atomic_t *p);