[PATCH] uml: close TUN/TAP file descriptors

When UML opens a TUN/TAP device, the file descriptor could be copied into
later, long-lived threads, holding the device open even after the interface is
taken down, preventing it from being brought up again.  This patch makes these
descriptors close-on-exec so that they disappear from helper processes, and
adds CLONE_FILES to a UML helper thread so that the descriptors are closed in
the thread when they are closed elsewhere in UML.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>