fileio: start with 4k buffer for procfs

fileio: start with 4k buffer for procfs

There's a very gradual increase of anonymous memory in systemd-journald that
blames to 2ac67221bb6270f0fbe7cbd0076653832cd49de2.

systemd-journald makes many calls to read /proc/PID/cmdline and
/proc/PID/status, both of which tend to be well under 4K. However the
combination of allocating 4M read buffers, then using `realloc()` to
shrink the buffer in `read_virtual_file()` appears to be creating
fragmentation in the heap (when combined with the other allocations
systemd-journald is doing).

To help mitigate this, try reading /proc with a 4K buffer as
`read_virtual_file()` did before 2ac67221bb6270f0fbe7cbd0076653832cd49de2.
If it isn't big enough then try again with the larger buffers.