s3:smbd: implement FSCTL_SET_SPARSE more correctly

s3:smbd: implement FSCTL_SET_SPARSE more correctly

this is a port of a patch from metze for 3.3:

We don't do the "strict allocation" when the sparse bit isn't
set, but that shouldn't matter.

We now allow windows applications to set and unset the sparse
bit.

Note that in order to implement this 100% like described
in [MS-FSA], we'd have to change our data model and support
the sparse flag per stream.