ia64: disable CONFIG_PERFMON at least for now

Al noted that the semantics of the ia64 perfmon mmap() is really wrong,
and can cause serious VM problems.  Before we look at whether it needs
to be fixed, let's see if anybody even uses it, and mark it broken.  It
may be that we can just remove the code entirely.

Reported-by: Alexander Viro <viro@zeniv.linux.org.uk>
Acked-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Stephane Eranian <eranian@google.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>