disk: Allow read hook callback to take read buffer to potentially modify it

It will be desirable in the future to allow having the read hook modify the
data passed back from a read function call on a disk or file. This adds that
infrastructure and has no impact on code flow for existing uses of the read
hook. Also changed is that now when the read hook callback is called it can
also indicate what error code should be sent back to the read caller.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/blocklist.c

@@ -53,9 +53,9 @@ print_blocklist (grub_disk_addr_t sector, unsigned num,
 }
 
 /* Helper for grub_cmd_blocklist.  */
-static void
+static grub_err_t
 read_blocklist (grub_disk_addr_t sector, unsigned offset, unsigned length,
-		void *data)
+		char *buf __attribute__ ((unused)), void *data)
 {
   struct blocklist_ctx *ctx = data;
 
@@ -70,7 +70,7 @@ read_blocklist (grub_disk_addr_t sector, unsigned offset, unsigned length,
 	}
 
       if (!length)
-	return;
+	return GRUB_ERR_NONE;
       print_blocklist (ctx->start_sector, ctx->num_sectors, 0, 0, ctx);
       ctx->num_sectors = 0;
     }
@@ -87,7 +87,7 @@ read_blocklist (grub_disk_addr_t sector, unsigned offset, unsigned length,
     }
 
   if (!length)
-    return;
+    return GRUB_ERR_NONE;
 
   if (length & (GRUB_DISK_SECTOR_SIZE - 1))
     {
@@ -103,6 +103,8 @@ read_blocklist (grub_disk_addr_t sector, unsigned offset, unsigned length,
       ctx->start_sector = sector;
       ctx->num_sectors = length >> GRUB_DISK_SECTOR_BITS;
     }
+
+  return GRUB_ERR_NONE;
 }
 
 static grub_err_t