security: Make LSM_MMAP_MIN_ADDR default match its help text.

Commit 788084aba2ab7348257597496befcbccabdc98a3 added the LSM_MMAP_MIN_ADDR
option, whose help text states "For most ia64, ppc64 and x86 users with lots
of address space a value of 65536 is reasonable and should cause no problems."
Which implies that it's default setting was typoed.

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>