- in mkdir: we already hold parent directory's semaphore (c.f. driverfs_create_dir)
- in create: ditto (c.f. driverfs_create_file)
- in unlink: ditto (c.f. driverfs_remove_file) and file's i_sem is taken in vfs_unlink
- un lseek: take inode's i_sem (though I think we can replace this with a common lseek function...later)