VFS: Add a fallthrough flag for marking virtual dentries

VFS: Add a fallthrough flag for marking virtual dentries

Add a DCACHE_FALLTHRU flag to indicate that, in a layered filesystem, this is
a virtual dentry that covers another one in a lower layer that should be used
instead.  This may be recorded on medium if directory integration is stored
there.

The flag can be set with d_set_fallthru() and tested with d_is_fallthru().

Original-author: Valerie Aurora <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: Al Viro <[email protected]>