Don't dump task struct in a.out core-dumps

akiphie points out that a.out core-dumps have that odd task struct
dumping that was never used and was never really a good idea (it goes
back into the mists of history, probably the original core-dumping
code). Just remove it.

Also do the access_ok() check on dump_write(). It probably doesn't
matter (since normal filesystems all seem to do it anyway), but he
points out that it's normally done by the VFS layer, so ...

[ I suspect that we should possibly do "vfs_write()" instead of
calling ->write directly. That also does the whole fsnotify and write
statistics thing, which may or may not be a good idea. ]

And just to be anal, do this all for the x86-64 32-bit a.out emulation
code too, even though it's not enabled (and won't currently even
compile)

Reported-by: akiphie <akiphie@lavabit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>