bootmem: Fix the short description of reserve_bootmem()

It marks pages as reserved, as the long description says.
Signed-off-by: Javi Merino <javi.merino@arm.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

mm/bootmem.c

@@ -419,7 +419,7 @@ int __init reserve_bootmem_node(pg_data_t *pgdat, unsigned long physaddr,
 }
 
 /**
- * reserve_bootmem - mark a page range as usable
+ * reserve_bootmem - mark a page range as reserved
  * @addr: starting address of the range
  * @size: size of the range in bytes
  * @flags: reservation flags (see linux/bootmem.h)