mm/memory.c: fail when offset == num in first check of __vm_map_pages()

If the caller asks us for offset == num, we should already fail in the
first check, i.e.  the one testing for offsets beyond the object.

At the moment, we are failing on the second test anyway, since count
cannot be 0.  Still, to agree with the comment of the first test, we
should first test it there.

Link: http://lkml.kernel.org/r/20190528193004.GA7744@gmail.com
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>