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/ubi/gluebi.c b/drivers/mtd/ubi/gluebi.c
index 941bc3c..0101dce 100644
--- a/drivers/mtd/ubi/gluebi.c
+++ b/drivers/mtd/ubi/gluebi.c
@@ -340,11 +340,11 @@
 	mtd->owner      = THIS_MODULE;
 	mtd->writesize  = di->min_io_size;
 	mtd->erasesize  = vi->usable_leb_size;
-	mtd->read       = gluebi_read;
-	mtd->write      = gluebi_write;
-	mtd->erase      = gluebi_erase;
-	mtd->get_device = gluebi_get_device;
-	mtd->put_device = gluebi_put_device;
+	mtd->_read       = gluebi_read;
+	mtd->_write      = gluebi_write;
+	mtd->_erase      = gluebi_erase;
+	mtd->_get_device = gluebi_get_device;
+	mtd->_put_device = gluebi_put_device;
 
 	/*
 	 * In case of dynamic a volume, MTD device size is just volume size. In