mm: do not call do_fault_around for non-linear fault

Ingo Korb reported that "repeated mapping of the same file on tmpfs
using remap_file_pages sometimes triggers a BUG at mm/filemap.c:202 when
the process exits".

He bisected the bug to d7c17551 ("mm: implement ->map_pages for
shmem/tmpfs"), although the bug was actually added by commit
8c6e50b0

 ("mm: introduce vm_ops->map_pages()").

The problem is caused by calling do_fault_around for a _non-linear_
fault.  In this case pgoff is shifted and might become negative during
calculation.

Faulting around non-linear page-fault makes no sense and breaks the
logic in do_fault_around because pgoff is shifted.

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Reported-by: Ingo Korb <ingo.korb@tu-dortmund.de>
Tested-by: Ingo Korb <ingo.korb@tu-dortmund.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Ning Qu <quning@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>	[3.15.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>