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

module/os/linux/zfs/arc_os.c

@@ -113,8 +113,7 @@ arc_free_memory(void)
 	return (ptob(si.freeram - si.freehigh));
 #else
 	return (ptob(nr_free_pages() +
-	    nr_inactive_file_pages() +
-	    nr_slab_reclaimable_pages()));
+	    nr_inactive_file_pages()));
 #endif /* CONFIG_HIGHMEM */
 }