files: Use sys_kcmp to find file descriptor duplicates v4

We switch generic-object-id concept with sys_kcmp approach,
which implies changes of image format a bit (and since it's
early time for project overall, we're allowed to).

In short -- previously every file descriptor had an ID
generated by a kernel and exported via procfs. If the
appropriate file descriptors were the same objects in
kernel memory -- the IDs did match up to bit. It allows
us to figure out which files were actually the identical
ones and should be restored in a special way.

Once sys_kcmp system call was merged into the kernel,
we've got a new opprotunity -- to use this syscall instead.
The syscall basically compares kernel objects and returns
ordered results suitable for objects sorting in a userspace.

For us it means -- we treat every file descriptor as a combination
of 'genid' and 'subid'. While 'genid' serves for fast comparison
between fds, the 'subid' is kind of a second key, which guarantees
uniqueness of genid+subid tuple over all file descritors found
in a process (or group of processes).

To be able to find and dump file descriptors in a single pass we
collect every fd into a global rbtree, where (!) each node might
become a root for a subtree as well.

The main tree carries only non-equal genid. If we find genid which
is already in tree, we need to make sure that it's either indeed
a duplicate or not. For this we use sys_kcmp syscall and if we
find that file descriptors are different -- we simply put new
fd into a subtree.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>

include/files.h

@@ -22,7 +22,7 @@ struct fmap_fd {
 };
 
 struct fdinfo_desc {
-	char			id[FD_ID_SIZE];
+	u64			id;
 	u64			addr;
 	int			pid;
 	u32			real_pid;	/* futex */