BACKPORT: FROMGIT: userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled

For background, mm/userfaultfd.c provides a general mcopy_atomic
implementation.  But some types of memory (i.e., hugetlb and shmem) need a
slightly different implementation, so they provide their own helpers for
this.  In other words, userfaultfd is the only caller of these functions.

This patch achieves two things:

1. Don't spend time compiling code which will end up never being
   referenced anyway (a small build time optimization).

2. In patches later in this series, we extend the signature of these
   helpers with UFFD-specific state (a mode enumeration).  Once this
   happens, we *have to* either not compile the helpers, or
   unconditionally define the UFFD-only state (which seems messier to me).
   This includes the declarations in the headers, as otherwise they'd
   yield warnings about implicitly defining the type of those arguments.

Link: https://lkml.kernel.org/r/20210301222728.176417-4-axelrasmussen@google.com


Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Adam Ruprecht <ruprecht@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Cannon Matthews <cannonmatthews@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michal Koutn" <mkoutny@suse.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shawn Anastasio <shawn@anastas.io>
Cc: Steven Price <steven.price@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

(cherry picked from commit 0e6e243e1d9a252c047c4cb1b032cfb31caf87ea
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git akpm)
Link: https://lore.kernel.org/patchwork/patch/1388133/



Conflicts:
	include/linux/hugetlb.h

(changed return type of hugetlb_reserve_pages from bool to int))
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Bug: 160737021
Bug: 169683130
Change-Id: I765cff74cde5fb4ce8141fb95e41848890ced961