Increase MALLOC_MP_LIM to 16MB

Increase the maximum allowed size passed to mprotect(PROT_WRITE)
from 1MB to 16MB. This was necessary with the glibc allocator
in order to allow worker threads to allocate more memory --
which in turn is necessary because of our new use of worker
threads for compression.

Closes ticket #22096. Found while working on #21648.

changes/bug22096

@@ -0,0 +1,6 @@
+  o Minor features (linux seccomp2 sandbox):
+    - Increase the maximum allowed size passed to mprotect(PROT_WRITE)
+      from 1MB to 16MB. This was necessary with the glibc allocator in
+      order to allow worker threads to allocate more memory -- which in
+      turn is necessary because of our new use of worker threads for
+      compression. Closes ticket 22096.

src/common/sandbox.c

@@ -20,7 +20,7 @@
 #endif
 
 /** Malloc mprotect limit in bytes. */
-#define MALLOC_MP_LIM 1048576
+#define MALLOC_MP_LIM (16*1024*1024)
 
 #include <stdio.h>
 #include <string.h>