security: enhance DEFAULT_MMAP_MIN_ADDR description

Got burned by setting the proposed default of 65536
across all Debian archs.

Thus proposing to be more specific on which archs you may
set this. Also propose a value for arm and friends that
doesn't break sshd.

Reword to mention working archs ia64 and ppc64 too.
Signed-off-by: maximilian attems <max@stro.at>
Cc: Martin Michlmayr <tbm@cyrius.com>
Cc: Gordon Farquharson <gordonfarquharson@gmail.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/Kconfig

@@ -113,10 +113,12 @@ config SECURITY_DEFAULT_MMAP_MIN_ADDR
 	  from userspace allocation.  Keeping a user from writing to low pages
 	  can help reduce the impact of kernel NULL pointer bugs.
 
-	  For most users with lots of address space a value of 65536 is
-	  reasonable and should cause no problems.  Programs which use vm86
-	  functionality would either need additional permissions from either
-	  the LSM or the capabilities module or have this protection disabled.
+	  For most ia64, ppc64 and x86 users with lots of address space
+	  a value of 65536 is reasonable and should cause no problems.
+	  On arm and other archs it should not be higher than 32768.
+	  Programs which use vm86 functionality would either need additional
+	  permissions from either the LSM or the capabilities module or have
+	  this protection disabled.
 
 	  This value can be changed after boot using the
 	  /proc/sys/vm/mmap_min_addr tunable.