Kernel: Annotate all syscalls with VERIFY_PROCESS_BIG_LOCK_ACQUIRED

Before we start disabling acquisition of the big process lock for
specific syscalls, make sure to document and assert that all the
lock is held during all syscalls.

Kernel/Syscalls/get_dir_entries.cpp

@@ -11,6 +11,7 @@ namespace Kernel {
 
 KResultOr<FlatPtr> Process::sys$get_dir_entries(int fd, Userspace<void*> user_buffer, size_t user_size)
 {
+    VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this);
     REQUIRE_PROMISE(stdio);
     if (user_size > NumericLimits<ssize_t>::max())
         return EINVAL;