mm: Support compiling out madvise and fadvise

Many embedded systems will not need these syscalls, and omitting them
saves space.  Add a new EXPERT config option CONFIG_ADVISE_SYSCALLS
(default y) to support compiling them out.

bloat-o-meter:
add/remove: 0/3 grow/shrink: 0/0 up/down: 0/-2250 (-2250)
function                                     old     new   delta
sys_fadvise64                                 57       -     -57
sys_fadvise64_64                             691       -    -691
sys_madvise                                 1502       -   -1502

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 391d4dd..d4709d4 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -156,6 +156,9 @@
 cond_syscall(compat_sys_process_vm_readv);
 cond_syscall(compat_sys_process_vm_writev);
 cond_syscall(sys_uselib);
+cond_syscall(sys_fadvise64);
+cond_syscall(sys_fadvise64_64);
+cond_syscall(sys_madvise);
 
 /* arch-specific weak syscall entries */
 cond_syscall(sys_pciconfig_read);