VFS: Add a whiteout dentry type

Add DCACHE_WHITEOUT_TYPE and provide a d_is_whiteout() accessor function.  A
d_is_miss() accessor is also added for ordinary cache misses and
d_is_negative() is modified to indicate either an ordinary miss or an enforced
miss (whiteout).

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>