mm/page_alloc.c: change mm debug routines back to EXPORT_SYMBOL

A new dump_page() routine was recently added, and marked
EXPORT_SYMBOL_GPL.  dump_page() was also added to the VM_BUG_ON_PAGE()
macro, and so the end result is that non-GPL code can no longer call
get_page() and a few other routines.

This only happens if the kernel was compiled with CONFIG_DEBUG_VM.

Change dump_page() to be EXPORT_SYMBOL.

Longer explanation:

Prior to commit 309381fe ("mm: dump page when hitting a VM_BUG_ON
using VM_BUG_ON_PAGE") , it was possible to build MIT-licensed (non-GPL)
drivers on Fedora.  Fedora is semi-unique, in that it sets
CONFIG_VM_DEBUG.

Because Fedora sets CONFIG_VM_DEBUG, they end up pulling in dump_page(),
via VM_BUG_ON_PAGE, via get_page().  As one of the authors of NVIDIA's
new, open source, "UVM-Lite" kernel module, I originally choose to use
the kernel's get_page() routine from within nvidia_uvm_page_cache.c,
because get_page() has always seemed to be very clearly intended for use
by non-GPL, driver code.

So I'm hoping that making get_page() widely accessible again will not be
too controversial.  We did check with Fedora first, and they responded
(https://bugzilla.redhat.com/show_bug.cgi?id=1074710#c3

) that we should
try to get upstream changed, before asking Fedora to change.  Their
reasoning seems beneficial to Linux: leaving CONFIG_DEBUG_VM set allows
Fedora to help catch mm bugs.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>