[Support/UNIX] posix_fallocate() can fail with EINVAL.

[Support/UNIX] posix_fallocate() can fail with EINVAL.

According to the docs on opegroup.org, the function can return
EINVAL if:

The len argument is less than zero, or the offset argument is less
than zero, or the underlying file system does not support this
operation.

I'd say it's a peculiar choice (when EONOTSUPP is right there), but
let's keep POSIX happy for now. This was independently discovered
by Mark Millard (on FreeBSD/ZFS).

Quickly ack'ed by Rui on IRC.

llvm-svn: 317535