c/r: prctl: fix build error for no-MMU case

Commit 1ad75b9e ("c/r: prctl: add minimal address test to
PR_SET_MM") added some address checking to prctl_set_mm() used by
checkpoint-restore.  This causes a build error for no-MMU systems:

   kernel/sys.c: In function 'prctl_set_mm':
   kernel/sys.c:1868:34: error: 'mmap_min_addr' undeclared (first use in this function)

The test for mmap_min_addr doesn't make a lot of sense for no-MMU code
as noted in commit 6e141546 ("NOMMU: Optimise away the
{dac_,}mmap_min_addr tests").

This patch defines mmap_min_addr as 0UL in the no-MMU case so that the
compiler will optimize away tests for "addr < mmap_min_addr".

Signed-off-by: Mark Salter <msalter@redhat.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: <stable@vger.kernel.org>	[3.6.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>