Fix fallocate error message.
Fix fallocate error message.

According to the documentation:

> RETURN VALUE
>       posix_fallocate() returns zero on success, or an error number on failure.  Note that errno is not set.

Since errno is not set by `posix_fallocate` potentially confusing error
messages can be printed since `h2o_perror` (called below) expects errno
to be set appropriately.

This change fixes this, so that h2o will print correct error messages if
`posix_fallocate` fails.

yannick/h2o,yannick/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,h2o/h2o