This patch uses stricter locking for REQ_LINK, REQ_MKDIR, REQ_MKNOD,
REQ_RENAME, REQ_RMDIR, REQ_SLINK and REQ_UNLINK. For all requests, VFS
locks the directory in which we add or remove an inode with VNODE_WRITE.
I.e., the operations have exclusive access to that directory.
Furthermore, REQ_CHOWN, REQ_CHMOD, and REQ_FTRUNC now lock the vmnt
VMNT_READ; VMNT_WRITE was unnecessary.