vfs_ceph: remove ceph_fallocate/ceph_ftruncate fallback

vfs_ceph: remove ceph_fallocate/ceph_ftruncate fallback

Both libcephfs functions are supported and capable of extending files,
so fallback can be dropped.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13807

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
(cherry picked from commit 155f1289ba7a4802fbb99fbc9ea90d8bc6cff0c9)
[ddiss@samba.org: rebase atop 48t without 532ff3a5b958]

Autobuild-User(v4-8-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-8-test): Tue Mar  5 13:01:15 UTC 2019 on sn-devel-144