syscall.Dirent declares a 256-byte "name" field. In reality, this
field is only as long as necessary to transmit the name, so more
entries can be packed in a page.

When casting the result to syscall.Dirent, the result might run over
the end of the byte buffer, which the race detector doesn't like.

Address this by using our own flavor of syscall.Dirent. All linux
flavors use the same structure layout, except for the amount of
padding at the end.

Change-Id: I618f7ef9e167894e1ab25b2113dcd20d7b76ceb4