On some libc (like musl[1]), posix_fallocate() is a fallocate() syscall
wrapper, and does not include fallback code like glibc does. In that
case, EOPNOTSUPP is returned if underlying filesystem does not
support fallocate() with mode = 0.

This patch enables falling back to writing zeros when EOPNOTSUPP, fixes
some cases like running on filesystem without proper fallocate support
on Alpine.

[1]: https://git.musl-libc.org/cgit/musl/tree/src/fcntl/posix_fallocate.c?h=v1.2.1