NFS: Fix WARN_ON due to unionization of nfs_inode.nrequests

Fixes the following WARN_ON
WARNING: CPU: 2 PID: 18678 at fs/nfs/inode.c:123 nfs_clear_inode+0x3b/0x50 [nfs]
...
Call Trace:
  nfs4_evict_inode+0x57/0x70 [nfsv4]
  evict+0xd1/0x180
  dispose_list+0x48/0x60
  evict_inodes+0x156/0x190
  generic_shutdown_super+0x37/0x110
  nfs_kill_super+0x1d/0x40 [nfs]
  deactivate_locked_super+0x36/0xa0
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/nfs_fs.h

@@ -573,7 +573,9 @@ bool nfs_commit_end(struct nfs_mds_commit_info *cinfo);
 static inline int
 nfs_have_writebacks(struct inode *inode)
 {
-	return atomic_long_read(&NFS_I(inode)->nrequests) != 0;
+	if (S_ISREG(inode->i_mode))
+		return atomic_long_read(&NFS_I(inode)->nrequests) != 0;
+	return 0;
 }
 
 /*