xtensa: don't make bootmem bitmap larger than required

If min_low_pfn is non-zero, the bitmap reserved for bootmem is bigger
than needed.  The number of pages bootmem has to maintain is the range
from min_low_pfn to max_low_pfn.

For now it has only been a theoretical mistake, min_low_pfn was always
zero.
Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/mm/init.c

@@ -130,7 +130,8 @@ void __init bootmem_init(void)
 
 	/* Find an area to use for the bootmem bitmap. */
 
-	bootmap_size = bootmem_bootmap_pages(max_low_pfn) << PAGE_SHIFT;
+	bootmap_size = bootmem_bootmap_pages(max_low_pfn - min_low_pfn);
+	bootmap_size <<= PAGE_SHIFT;
 	bootmap_start = ~0;
 
 	for (i=0; i<sysmem.nr_banks; i++)