RHEL4 defaults nfs export to "sync" so nfsd would invokes nfsd_sync_dir()
    that eventually calls gfs_fsync() with filp set to NULL. In lehman patch,
    I obtained inode location from filp, not knowing it could be set NULL by
    nfsd's vfs layer. This ends up crashing the kernel as described in
    bugzilla 193817.
    
    Fix the panic issue by getting inode pointer from dentry instead.