common: posix_fallocate on ZFS returns EINVAL

common: posix_fallocate on ZFS returns EINVAL

But even still it would not work on any COW FS.
So reorganised the code to have a common routine
that in the end will allocate a file on disk if needed

FileStore would not build when there was no HAVE_POSIX_FALLOCATE
other than on Apple. With ceph_posix_fallocate FileStore will also
fallback to manually allocating the required file.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>