Linux 5.9 compat: NR_SLAB_RECLAIMABLE

Commit dcdc12e added compatibility code to treat NR_SLAB_RECLAIMABLE_B
as if it were the same as NR_SLAB_RECLAIMABLE. However, the new value
is in bytes while the old value was in pages which means they are not
interchangeable.

The only place the reclaimable slab size is used is as a component of
the calculation done by arc_free_memory(). This function returns the
amount of memory the ARC considers to be free or reclaimable at little
cost. Rather than switch to a new interface to get this value it has
been removed it from the calculation. It is normally a minor component
compared to the number of inactive or free pages, and removing it
aligns the behavior with the FreeBSD version of arc_free_memory().

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Coleman Kane <ckane@colemankane.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10834