NFS: Provide stub nfs_fscache_wait_on_invalidate() for when CONFIG_NFS_FSCACHE=n

Provide a stub nfs_fscache_wait_on_invalidate() function for when
CONFIG_NFS_FSCACHE=n lest the following error appear:

  fs/nfs/inode.c: In function 'nfs_invalidate_mapping':
  fs/nfs/inode.c:887:2: error: implicit declaration of function 'nfs_fscache_wait_on_invalidate' [-Werror=implicit-function-declaration]
  cc1: some warnings being treated as errors

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfs/fscache.h

@@ -222,6 +222,7 @@ static inline void nfs_readpage_to_fscache(struct inode *inode,
 
 
 static inline void nfs_fscache_invalidate(struct inode *inode) {}
+static inline void nfs_fscache_wait_on_invalidate(struct inode *inode) {}
 
 static inline const char *nfs_server_fscache_state(struct nfs_server *server)
 {