Bug#77857: hurd: write_node assertion failed building emacs

[Roland McGrath]

It looks to me like the ext2fs block allocation code (getblk.c) can set
dn_set_mtime when called from the pager whiled not holding the node lock.
It will hold node->dn->alloc_lock, but that is all.

The ext2_getblk code is called sometimes with the node lock held
(storeinfo.c, diskfs_grow) and sometimes without (pager).  So it cannot
just take the lock around setting dn_set_?time.  I'm not sure whether the
node lock really ought to be held around setting them or not.  From reading
the ufs code, I cna't tell whether it has the same bug, but I think it does.