nodeMap.Handle(&node) == 0 means the kernel does not currently have an
entry for this node in its dentry cache. And the kernel would return
ENOENT if we would try to send it notify store message with nodeID it
does not currently know.

So return proper ENOENT when we see that currently there is no handle
for the inode, instead of returning EINVAL, which means that call
arguments - e.g. off is invalid.

NOTE contrary to cache retrieve (where we could turn such error into empty OK
read; see previous patch) for cache store we cannot ignore the error.

Fixes bdca0e6a (Add support for store notify).
See also: https://github.com/hanwen/go-fuse/pull/246#discussion_r242050595