safe_read_from: bump default size limit to 512k

safe_read_from: bump default size limit to 512k

this is (via file_get_contents and file_copy) used for reading from
pmxcfs, which has a file size limit of 512k. since quite a number of
call sites would need to explicitly override this (and then get updated
if we bump the limit on the pmxcfs side again in the future), making our
default file reader compatible by default seems the better solution.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>