Allow core_pattern pipes to wait for user space to complete

One of the things that user space processes like to do is look at metadata
for a crashing process in their /proc/<pid> directory.  this is racy
however, since do_coredump in the kernel doesn't wait for the user space
process to complete before it reaps the crashing process.  This patch
corrects that.  Allowing the kernel to wait for the user space process to
complete before cleaning up the crashing process.  This is a bit tricky to
do for a few reasons:

1) The user space process isn't our child, so we can't sys_wait4 on it
2) We need to close the pipe before waiting for the user process to complete,
since the user process may rely on an EOF condition

I've discussed several solutions with Oleg Nesterov off-list about this,
and this is the one we've come up with.  We add ourselves as a pipe reader
(to prevent premature cleanup of the pipe_inode_info), and remove
ourselves as a writer (to provide an EOF condition to the writer in user
space), then we iterate until the user space process exits (which we
detect by pipe->readers == 1, hence the > 1 check in the loop).  When we
exit the loop, we restore the proper reader/writer values, then we return
and let filp_close in do_coredump clean up the pipe data properly.
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Earl Chew <earl_chew@agilent.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>