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/mtdchar.c b/drivers/mtd/mtdchar.c
index 50c6a1e..426640e 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -405,7 +405,7 @@
 	if (length > 4096)
 		return -EINVAL;
 
-	if (!mtd->write_oob)
+	if (!mtd->_write_oob)
 		ret = -EOPNOTSUPP;
 	else
 		ret = access_ok(VERIFY_READ, ptr, length) ? 0 : -EFAULT;
@@ -576,7 +576,7 @@
 			!access_ok(VERIFY_READ, req.usr_data, req.len) ||
 			!access_ok(VERIFY_READ, req.usr_oob, req.ooblen))
 		return -EFAULT;
-	if (!mtd->write_oob)
+	if (!mtd->_write_oob)
 		return -EOPNOTSUPP;
 
 	ops.mode = req.mode;