gdb/linux-record: Fix readdir and getdents handling
getdents buffer size is given in bytes, not dirent entries (which have
variable size anyway). We don't need size_dirent and size_dirent64 for
this reason.
readdir, on the other hand, needs size of old_linux_dirent, which is
a somewhat different structure. Accordingly, rename size_dirent
to size_old_dirent.