drivers/mtd/ftl: bypass rwbuffer when O_DIRECT is set

ftl_write() unconditionally routes writes through rwb_write() when
FTL_HAVE_RWBUFFER is defined, ignoring the O_DIRECT flag stored in
dev->oflags. This means callers that open with O_DIRECT expecting
unbuffered writes still get their data buffered in RAM.

The O_DIRECT flag is already checked in ftl_flush() to select
between direct and read-modify-write paths, but the buffering
decision in ftl_write() never consults it.

Add O_DIRECT checks in both ftl_write() and ftl_read() to bypass
rwbuffer entirely when the flag is set. For writes, go directly to
ftl_flush() and normalize the return value to match the block
driver convention (nsectors on success). For reads, go directly to
ftl_reload(), bypassing any read-ahead cache.

This gives O_DIRECT callers a complete bypass of the rwbuffer layer
for both reads and writes, matching the documented contract.

Signed-off-by: Neil Berkman <neil@xuku.com>

drivers/mtd/ftl.c

@@ -455,6 +455,11 @@ static ssize_t ftl_read(FAR struct inode *inode, unsigned char *buffer,
 
   dev = inode->i_private;
 #ifdef FTL_HAVE_RWBUFFER
+  if (dev->oflags & O_DIRECT)
+    {
+      return ftl_reload(dev, buffer, start_sector, nsectors);
+    }
+
   return rwb_read(&dev->rwb, start_sector, nsectors, buffer);
 #else
   return ftl_reload(dev, buffer, start_sector, nsectors);
@@ -759,6 +764,12 @@ static ssize_t ftl_write(FAR struct inode *inode,
   DEBUGASSERT(inode->i_private);
   dev = inode->i_private;
 #ifdef FTL_HAVE_RWBUFFER
+  if (dev->oflags & O_DIRECT)
+    {
+      ssize_t ret = ftl_flush(dev, buffer, start_sector, nsectors);
+      return ret < 0 ? ret : nsectors;
+    }
+
   return rwb_write(&dev->rwb, start_sector, nsectors, buffer);
 #else
   return ftl_flush(dev, buffer, start_sector, nsectors);