ceph: Fix incorrect flush end position calculation

In ceph, in fill_fscrypt_truncate(), the end flush position is calculated
by:

                loff_t lend = orig_pos + CEPH_FSCRYPT_BLOCK_SHIFT - 1;

but that's using the block shift not the block size.

Fix this to use the block size instead.

Fixes: 5c64737d ("ceph: add truncate size handling support for fscrypt")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>