linux(4): Properly allocate buffer for kern_getdirentries in readdir

Looks like prior to ino64 project the size of the struct linux_dirent
was greater (or equal) to the size of the native struct dirent so the
native dirent fit into the buffer. After ino64 project the size of the
native struct dirent has increased.

Spotted by gcc12.
MFC after: 2 weeks