mtd: add leading underscore to all mtd functions

This patch renames all MTD functions by adding a "_" prefix:

mtd->erase -> mtd->_erase
mtd->read_oob -> mtd->_read_oob
...

The reason is that we are re-working the MTD API and from now on it is
an error to use MTD function pointers directly - we have a corresponding
API call for every pointer. By adding a leading "_" we achieve the following:

1. Make sure we convert every direct pointer users
2. A leading "_" suggests that this interface is internal and it becomes
   less likely that people will use them directly
3. Make sure all the out-of-tree modules stop compiling and the owners
   spot the big API change and amend them.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/chips/map_absent.c b/drivers/mtd/chips/map_absent.c
index f2b8729..6be2eddf 100644
--- a/drivers/mtd/chips/map_absent.c
+++ b/drivers/mtd/chips/map_absent.c
@@ -55,10 +55,10 @@
 	mtd->name 	= map->name;
 	mtd->type 	= MTD_ABSENT;
 	mtd->size 	= map->size;
-	mtd->erase 	= map_absent_erase;
-	mtd->read 	= map_absent_read;
-	mtd->write 	= map_absent_write;
-	mtd->sync 	= map_absent_sync;
+	mtd->_erase 	= map_absent_erase;
+	mtd->_read 	= map_absent_read;
+	mtd->_write 	= map_absent_write;
+	mtd->_sync 	= map_absent_sync;
 	mtd->flags 	= 0;
 	mtd->erasesize  = PAGE_SIZE;
 	mtd->writesize  = 1;