Don't fadvise we don't need data that we do need

Don't fadvise we don't need data that we do need

The documentation for posix_fadvise states that if len is 0 the advice
extends to the end of the file. We currently always pass 0 for
the first two posix_fadvise calls (first because ofs is 0, second
because ofs == BLOB_READ_SIZE == 1024*1024) so we're advising the kernel
to dump any predicted file caching twice per file.

This patch ensures we don't pass a len of 0 in the two scenarios above.

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>