gluster: Query current size in do_truncate()

Instead of expecting the current size to be 0, query it and allocate
only the area [current_size, offset) if preallocation is requested.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
1 file changed