pid1,nspawn: raise default RLIMIT_MEMLOCK to 8M

This mirrors a similar check in Linux kernel 5.16
(9dcc38e2813e0cd3b195940c98b181ce6ede8f20) that raised the
RLIMIT_MEMLOCK to 8M.

This change does two things: raise the default limit for nspawn
containers (where we try to mimic closely what the kernel does), and
bump it when running on old kernels which still have the lower setting.

Fixes: #16300
See: https://lwn.net/Articles/876288/

src/basic/def.h

@@ -57,8 +57,13 @@
 #define CONF_PATHS_STRV(n)                      \
         STRV_MAKE(CONF_PATHS(n))
 
+/* The limit for PID 1 itself (which is not inherited to children) */
 #define HIGH_RLIMIT_MEMLOCK (1024ULL*1024ULL*64ULL)
 
+/* Since kernel 5.16 the kernel default limit was raised to 8M. Let's adjust things on old kernels too, and
+ * in containers so that our children inherit that. */
+#define DEFAULT_RLIMIT_MEMLOCK (1024ULL*1024ULL*8ULL)
+
 #define PLYMOUTH_SOCKET {                                       \
                 .un.sun_family = AF_UNIX,                       \
                 .un.sun_path = "\0/org/freedesktop/plymouthd",  \