Reduces Sizeof(nodefs.Inode{}) from 112 to 96 bytes.

The "generation" field in "Inode" is not used anywhere.
"portableHandleMap" has it's own generation field, which
is actually used.

The "check" field, embedded through the "handled" struct,
is checked for zero, but never set.

This is part of an effort to reduce the memory usage of go-fuse
and gocryptfs. Details can be seen at
https://github.com/rfjakob/gocryptfs/issues/132 .