Another fix for the __LOW macro.

When dm_table and dm_target structures are initialized, the "limits" fields
(struct io_restrictions) are initialized to zero (e.g. in dm_table_add_target()
in dm-table.c). However, zero is not a useable value in these fields. The
request queue will never let an I/O through, regardless of how small it might
be, if max_sectors is set to zero (see generic_make_request in ll_rw_blk.c).
This change to the __LOW() macro sets these fields correctly when they are
first initialized.  [Kevin Corry]