epoll: support for disabling items, and a self-test app

epoll: support for disabling items, and a self-test app

Enhanced epoll_ctl to support EPOLL_CTL_DISABLE, which disables an epoll
item.  If epoll_ctl doesn't return -EBUSY in this case, it is then safe to
delete the epoll item in a multi-threaded environment.  Also added a new
test_epoll self- test app to both demonstrate the need for this feature
and test it.

Signed-off-by: Paton J. Lewis <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jason Baron <[email protected]>
Cc: Paul Holland <[email protected]>
Cc: Davide Libenzi <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>