These are tied together a bit, so they're included in the same patch

Mainly, they move the taking of the inode's i_sem into the unlink and rmdir. 

driverfs_rmdir doesn't call driverfs_unlink anymore, as it checks if the directory is empty
and conditionally does d_delete on it.

fs/namei.c implements d_unhash, which is called in vfs_rmdir. This isn't exported (yet), 
so reimplement it here (at least until it's known that it's not needed or it's exported).