nspawn: Include arm_fadvise64_64 in syscall allow_list

Add the `arm_fadvise64_64` syscall to the allow_list, in addition
to the existing `fadvise64` and `fadvise64_64` syscalls, as this is
the syscall actually defined for `arm` architecture.  Adding it fixes
the syscall being rejected in arm32 containers.

Fixes #35194

(cherry picked from commit 7fd70a5)
(cherry picked from commit 964ced4)

Author: mgorny

src/nspawn/nspawn-seccomp.c

@@ -50,6 +50,7 @@ static int add_syscall_filters(
                 { CAP_IPC_LOCK,       "@memlock"                     },
 
                 /* Plus a good set of additional syscalls which are not part of any of the groups above */
+                { 0,                  "arm_fadvise64_64"             },
                 { 0,                  "brk"                          },
                 { 0,                  "capget"                       },
                 { 0,                  "capset"                       },