implement InvalidateNode/InvalidateEntry

[ikonst]

Implement fuse_lowlevel_notify_inval_inode. This should allow user space processes to trigger eviction of the kernel's inode cache, and in turn getting the "ForgetInodeOp" to evict its own data.

Context: https://sourceforge.net/p/fuse/mailman/message/30057855/

[riking]

I don't like the way the PR is approaching this. We should come up with a general user API for all the inotify sending, then some helpers to generate "please forget inode" notifications.