mincore: fix reading information

mincore: fix reading information

The _fmincore_chunk_size is typically set to 64MiB, which
makes sense to avoid doing very large mmap() operations
(to save already precious VM on 32-bit systems).

However, since that's in bytes, we cannot divide a size in
pages by it, and expect any useful outcome.

Calculate the number of chunks (chunk_count) properly based
on the size of the file, rather than its number of pages.
Otherwise, chunk_count typically ends up just 1 even for a
very large file (my test file was ~500MiB), and mincore()
is run just once, so we fill the presence information only
for the first 64MiB of the file, even if it was previously
completely in RAM.

Given a large enough test file (and enough RAM to keep it
there), the following should print about the same times
twice:

  cat test > /dev/null ; \
  time cat test > /dev/null ; \
  bup split --noop test ; \
  time cat test >/dev/null

Without the fix, it's evident that the file is evicted from
RAM almost entirely (apart from the first 64MiB) even in
this synthetic case.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>