Kernel: Don't copy a Vector<FileDescriptionAndFlags>

Instead of copying a Vector everytime we need to enumerate a Process'
file descriptions, we can just temporarily lock so it won't change.

Kernel/Syscalls/lseek.cpp

@@ -12,7 +12,7 @@ namespace Kernel {
 KResultOr<FlatPtr> Process::sys$lseek(int fd, Userspace<off_t*> userspace_offset, int whence)
 {
     REQUIRE_PROMISE(stdio);
-    auto description = file_description(fd);
+    auto description = fds().file_description(fd);
     if (!description)
         return EBADF;
     off_t offset;